Introduction - University of Nevada, Las Vegas

1

Java Programming

Java Syntax from the Ground Up

CSC 140 Java Programming © Matt B. Pedersen ([email protected]) 2

Introduction


Lexical Structure -Unicode Character Set

! Most languages use a 7/8 bit ASCII codetable for encoding (128/256 differentcharacters)


Lexical Structure - Unicode

! Unicode is 16 bit (32,768 differentcharacters)! Special unicode editor might be required! or, encode characters as \uxxxx

! xxxx is in hex! Examples:

! \ux0020 is space! \u03c0 is !


Lexical Structure -Case-Sensitivity and Whitespace

! Java is cAse-SEnsitIve ;-)! While, WHILE, and while are different

things! While and WHILE are identifiers.! while is a reserved word.

! If you have declared a variable i you cannotrefer to it as I.

! Whitespace is ignored (spaces, tabs andnewlines)


Comments

! Comments are ignored by the compiler;there are three different types ofcomments:! Single line comment ( //… )int i = 0; // initialize the loop variable

! Multi-line comments (type 1, /* … */)/* * First, establish a connection to the server. * If the connection attempt fails, quit right away */


Comments

! Multi-line comments (type 2, /** … */,doc-comment) used for generatingautomatic Javadoc documentation/** * Upload a file to a web server * * @param file The file to upload * @return <tt>true</tt> on success. * <tt>false</tt> on failures. * @author David Flanagan */


Generated Javadoc


Reserved Words

thisprotectedinstanceoffalseclass

whilesyncrhonizedprivateimportextendschar

volatileswitchpackageimplementsenumcatch

voidsupernullifelsecase

trystrictfpnewgotodoublebyte

truestaticnativefordobreak

transientshortlongfloatdefaultboolean

throwsreturninterfacefinallycontinueassert

throwpublicintfinalconstabstract


Reserved Words (Modifiers)









throwpublicintfinalconst abstract


Reserved Words (Types)











Reserved Words (Exceptions)











Reserved Words (Control Flow)











Reserved Words (Literals)











Reserved Words (Modularity)











Reserved Words (OO stuff)











Identifiers


Literals

! Integer and floating point numbers,characters, strings, as well as true,false and null

1, 1.0, 1f, 1L, ‘1’, “one”, true, null

(int, double, float, long, char, string, boolean, class type)


Punctuation

! Separators( ) { } [ ] < > : ; , . @

! Operators+ - * / % & | ^ << >> >>>+= -= *= /= %= &= |= ^= <<= >>= >>>== == != < <= > >=! ~ && || ++ ? :


Primitive Data Types! Java supports 8 primitive types

+/-4.9E-324 to+/-1.7976931348623157E+308

64 bit0.00IEEE 754 floatingpoint

double

+/-1.4E-45 to +/-3.4028235E+3832 bit0.0IEEE 754 floatingpoint

float

-9,223,372,036,854,775,808 to 9,223,372,036,854,775,80764 bit0Signed integerlong

-2,147,483,648 to 2,147,483,64732 bit0Signed integerint

-32,768 to 3276716 bit0Signed integershort

-128 to 1278 bits0Signed integerbyte

\u0000 to \uffff16 bits\u0000Unicode characterchar

N/A1 bitfalsetrue or falseboolean

RangeSizeDefaultContainsType


The boolean Type

! Represents a truth value.! Only two possible values:true or false

! Not like C where 0 is false and anythingelse is true.

! Cannot be compared to integers.


The char Type

! Represents unicode characterschar c = ’A’;char tab = ’\t’;char apostrophe = ’\’’;char nul = ’\000’;char aleph = ’\u05D0’;

! Note the difference here:char \u05D0 = ’\u05D0’;

\u05D0 is a variable name is unicode‘\u05D0’ is a character literal inunicode.


The char Type

Unicode character xxxx hexadecimal value\uxxxxLatin-1 character xxx octal value (0 to 377)\xxx/Backslash\\Single quote\’Double quote\”Carriage return\rForm feed\fNewline\nHorizontal tab\tBackspace\bCharacter valueEscape sequence


Strings

! String is not a primitive type, but aclass.

! Java does allow string Literals”Hello, World””This is a string!”

! Or some code:String greeting = “Hello stranger!”;String \u03c0 = "\"\'\u03c0\'\"";

delcares a string ”’π’” in a unicodevariable called \u03c0


Integer Types

! byte, short, integer, and long areall signed integer types (no unsignedinteger types exist in java)! 255 (decimal) = 0xff (hex) = 0377 (octal)! 1234 is a int value, 1234L is a long value


Overflow

! Neither the compiler nor the runtimesystem catches overflow

byte b1 = 127, b2 = 1;byte sum = (byte)(b1 + b2);

! What is the value of sum?


Wrapper Classes

! Classes for each integer type exist:Byte, Short, Integer, Long(Note the capital letter)

Byte.MAX_VALUEByte.MIN_VALUE

gives the max/min byte values


Floating-Point Types

! Real numbers in Java are representedby the float and double data types.

! 123.45! 0.0! .01! 1.2345E02 // 1.2345*102, or 123.45! 1e-6 // 1*106, or 0.000001



! Just like integer values are int types bydefault, floating point values are doubleby default.

double d = 6.02E23;float f = 6.02E23f;



! int i = 1/0 causes an exception,division by 0 is illegal in integerarithmetic.

! double inf = 1.0/0.0 does notcause an exception.



! 4 Special ‘values’ for floating points:

double inf = 1.0/0.0; // prints Infinitydouble neginf = -1.0/0.0; // prints -Infinitydouble negzero = -1.0/inf; // prints -0double NaN = 0.0/0.0; // prints NaN

! Infinity + anything = Infinity! -0 == 0! NaN is not equal to anything (incl. itself)! Double.isNaN() checks for NaN


Primitive Type Conversions

! Conversion is legal between integer andfloating points (incl. chars, but notboolean)! Widening: A value is converted to a type

with a larger range of legal values:double d = 10;

double is a ‘wider’ type than int.! Widening is always allowed



! Narrowing: Converting a value to a type thathas a smaller range of legal values.! Ok sometimes: int value 13 can be converted to

a byte, but 1300 cannot (byte range is -128 to127)

! Narrowing conversions can be forced by a cast.

int i = 13;byte b = (byte)i; // force int to be converted to a bytei = (int)13.456; // force this double literal to the int 13



---CCCCCCNdouble

Y---CCCCCNfloat

Y*Y*---CCCCNlong

YY*Y---CCCNint

YYYY---CCNchar

YYYYC---CNshort

YYYYCY---Nbyte

NNNNNNN---booleandoublefloatlongintcharshortbyteboolean

ToFrom

Y = yes, the conversion is a widening.N = not, the conversion cannot occur.C = yes, the conversion must be a narrowing cast.Y* = yes, by automatic widening that can cause loss of precision. This can happen when converting int and long to float and double.


Expressions and Operators

! The Java interpreter evaluatesexpressions to compute its value.

! The simplest expressions are calledprimary expressions, and consists ofliterals and variables: 1.7 // a floating point literal

true // a boolean literalsum // a variable



! The value of a literal is the value itself:The value of 1.7 is 1.7!

! The value of a variable is the valuestored in the location in memoryrepresented by that variable



! We use operators together with primaryexpressions to form more interestingexpressions:

sum = 1.7sum = 1 + 2 +3*1.2 + (4+8)/3.0 sum/Math.sqrt(3.0 * 1.234)(int)(sum + 33)

! Note, all valid expressions; assignments arenot statements but expressions, they have avalue.


Operator Summary

Left shift ( * 2)Right shift w/sign extension ( / 2)Right shift w/zero extension

Integer, integerInteger, integerInteger, integer

<<>>>>>

L10

Addition, subtractionString concatenation

Number, numberString, any

+,-+

L11

Multiplication, division, remainderNumber, number*,/,%L12

Object creationCast (type conversion)

Class, anyType, any

new( type )

R13

Pre increment/decrementUnary plus/minusBitwise complementBoolean NOT

VariableNumberIntegerBoolean

++/--+/-~!

R14

Object member accessArray element accessMethod invocationPost increment/decrement

Object, memberArray, intMethod, arglistVariable

.[ ]( args )++/--

L15

Operation PerformedOperand(s) typesOperatorAP


Operator Summary

Conditional ORBoolean, boolean||L3

Conditional ANDBoolean, boolean&&L4

Bitwise ORBoolean OR

Integer, integerBoolean, boolean

||

L5

Bitwise XORBoolean XOR


^^

L6

Bitwise ANDBoolean AND


&&

L7

Equal (have identical values)Not equal (have different values)Equal (refer to the same object)Not equal (refer to different objects)

Primitive, primitivePrimitive, primitiveReference, referenceReference, reference

==!===!=

L8

Less than, less than or equalGreater than, greater than or equalType comparison

Number, numberNumber, numberReference, type

<, <=>, >=Instanceof

L9



Operator Summary

AssignmentAssignment with operation

Variable, anyVariable, any

=*=, /=, %=,+=, -=, <<=,>>=, >>>=,&=, ^=, |=

R1

Conditional (ternary) operatorBoolean, any? :R2



Precedence

! Precedence specifies the order in whichoperations are performed. Eg.:

1 + 2 * 3 // == 7* has higher precedence than +, and isperformed first. Precedence can beoverruled using ( ):

(1 + 2) * 3 // == 9


Associativity

! Associativity governs the order ofoperation when we use operators of thesame precedence.! a = b += c = -~d! is: a = (b += (c = -(~d)))! Precedence can be overridden using ( )


Operand Number and Type

! Operators take different number ofoperands:! Unary operators take one operand

! -8 (unary minus)! ~i (bitwise complement)

! Binary operators take two operands! a + 8 (binary plus)! a << 8 (binary left shift)



! Operators take different number ofoperands:! Ternary operators take three operands

! x > y ? x : y (called a ternary expression)! Like an ‘if’-statement, but as an expression! min = (x > y) ? x : y



! Not all operators work on all types.! -”Hello”

(you cannot negate a string)! a + false

(+ does not work with booleans)! x > y ? “x is greater” : y

(Why is this one wrong? ;-) )


Return Type

! Just as an operator only works onspecific types, it also returns a value of aspecific type (sometimes dependent onthe type of the operands)! 1 + 1 = 2 (int + int = int)! 1.0 + 2.0 = 3.0

(double + double = double)! 1 + 2.0 = ???


Return Type

! Not all operators return values of thesame type as their operands:! a < 0 is of boolean type! x < y ? “x is smaller” : “y issmaller”always returns values of string type, but xand y can be any type compatible with <.


Side Effects

int a=2;a = ++a + ++a * a++;


Order of Evaluation

! The order in which operands areevaluated is always left to right! Taking into account

! Associativity of operator! Precedense of operator! Parentehsis ( )


Order of Evaluation

! Some operators do not evaluate all theiroperands:! x ? y : z (Ternary), only x and the

chosen branch are evaluated! x && y. If x is false there is no need to

evaluate y.! x || y. if x is true there is no need to

evaluate y.


Order of Evaluation

int a = 2;int v = ++a + ++a * ++a;

What is the value of v after theassignment?


Arithmetic Operators

! Addition (+)! Normally + adds numbers:

! 2 + 2 = 4 (integer + integer = integer)! 2 + 2.0 = 4.0 (integer + double = double)

! Can be used on strings:! “Hello “ + “world” = “Hello world”

! Be careful when mixing strings and numbers! “Total: “ + 3 + 4 = “Total: 34”! 3 + 4 + “ is the total” = “7 is the total”


Arithmetic Operators

! Other arithmetic operators:! Subtraction (-), both binary and unary! Multiplication (*)! Division (/ )

! 7/3 = 2! 7/3.0f = 2.3333333! 7/3.0 = 2.3333333333333335! 7/0 causes a runtime error (An exception)! 7/0.0 evaluates to ! (positive infinity)! 0.0/0.0 evaluates to NaN (Not a Number)


String Concatenation Operator

! Reminder: + and += can be used withstrings.

! Objects are converted to strings with thetoString() method(more about that later)

! “Once a string, always a string”:remember to use ( ) around arithmeticwhen mixed with strings.


Increment/Decrement Operators

! ++ or -- increment/decrement its singleoperand by 1.i = 1;j= i++;Is equivalent toi = 1;j = i + 1;



! a++ is not the same as ++a.! a++ uses the value of a then incrementint a = 1;System.out.println(a++); // prints 2

! ++a increments the value of a then uses itint a = 1;System.out.println(++a); // prints 1



! Be REAL careful about the ++ and --operators.

! Most of time x++ is equivalent tox = x + 1

! But not always:a[i++]++;a[i++] = a[i++] + 1;


Comparison Operators

! Used to test equality and size (order)! == equal (values!)

! Eg. a == b tests is the value stored in a is thesame as the value stored in b

! != not equal! a != b is the same as !(a == b)

! Careful: = is not a test for equality, it isassignment



! <, >, <=, >=! For comparing values that are ordered

! !=, == works on ‘anything’! <, >, <=, >= works on numbers only

! The result of all of these operators is aboolean value: either true or false



Example:boolean b;int a, c;a = 10;c = 4;

b = !(a > c);

What is the value of b?


Boolean Operators

! A number of operators work on booleanvalues. They are either binary (two operands)or unary (one operand), but all produce aboolean result:! && Conditional AND

! true && true == true! true && false == false! false && false == false! false && true == false

Example: (x < 10 && y > 3)


Boolean Operators

! || Conditional OR! true || true == true! true || false == true! false || false == false! false || true == true

! ! Boolean NOT! !true == false! !false == true


Short Circuit Boolean Evaluation

! Remember: Evaluation from LEFT to RIGHT,and with SCBE.

! To avoid SCBE and force evaluation of alloperand expressions use & and | instead.

! & and | used with integer operands computesa bit-wise operation. and | used with integeroperands computes a bit-wise operation.


Boolean Operators

! ^ is exclusive or (XOR)! true ^ true == false! true ^ false == true! false ^ true = true! false ^ false = false

! Can be used on integers as well! With boolean operands: ^ is the same as!=


Bitwise Operators

! Used to manipulate the individual bits ofintegral values/variables:! ~ bitwise complement! & bitwise AND (we already saw this with

booleans)! | bitwise OR! ^ bitwise XOR


Shift Operators

! Shift operators moves bits left or right:! << Left Shift:

10 << 1 // 00001010 << 1 == 00010100 = 20 = 10*2

! >> Signed Right Shift:27 >> 3 // 00011011 >> 3 == 00000011 = 3 /27/(2*2*2)-50 >> 2 // 11001110 >> 2 == 11110011 = -13 = -50/(2*2)

! >>> Unsigned Right Shift:-50 >>> 2 // 0xFFFFFFCE >>> 2 = 0x3FFFFFF3


Assignment Operator

! We have already see this one:! = (and not == which is comparison)

! Others exist:! += for example: a += 2; which is the same

as a = a + 2;! Others: -=, *=, /=, %=, &=, |=,^=, <<=, >>=, >>>=


The Conditional Operator

! The only operator to take three operands:! … ? … : …Example:a > 10 ? b+1 : c-2

! If a>10 then the entire thing evaluates to the valueof the expression b+1, else the value of c-2

! Really is an if-expression (as opposed to an ifstatement.

! Can be used as an expression: d = a>2 ? 4 :8;

! The two last operand expressions must be thesame type


The Conditional Operator

! Example of the conditional operator! Compute max value of x and y and assign to z;

if (x > y) z = x;else z = y;

! Orz = (x > y) ? x : y;


The instanceof Operator

! We will return to this one later! Determines if the left hand side object expression is

an object of a certain class type:! o instanceof A

! True if o is an object based on class A.

! Note, if o instanceof A is true, and B is asuper class of A, o instanceof B will also betrue.

! Tells us when it is safe to ‘cast’ an object referenceto a different class type.


Special Operators

! A number exist, let’s just considerexplicit casting! Explicit casting is “changing the type of a

value”(byte) 28(float) 3.14

! Objects can be cast as well.


Statements

! A Statement is a single command that has noreturn value. They include! Expression statements (Expressions where return values are

ignored)! Compound (block) statements: { … }! Empty statement: ;! Local variable declarations: int a;! If/else statement: if (…) … else …! Switch statement: switch (…) { case ..: …. ….. }! While statement: while (…) …! Do statement: do … while (…)! For statement: for (…;…;…) …! Break- / continue- / return-statement


Expression Statements

! An expression statement is anexpression used as a statement (valueof expression is ignored)a = 1;a *= 2;--c;f(7);


Compound Statements

a = 10; b = 1; while (a > 0) a = a - 1; b = b * 2;

What is the value of a and bafter the loop?


Compound Statements

a = 10; b = 1; while (a > 0) a = a - 1; b = b * 2;

What is the value of a and bafter the loop?

a = 0b = 2


Compound Statements

! A compound statement is a number ofstatements grouped together using { }

a = 10;b = 1;

while (a > 0) { b = b * 2; a = a - 1;

}


The Empty Statement

! The empty statement is just a semicolon:;

for (int i=0; i<10; a[i++]++) ;

! Rarely a use for it! The above exampleshould be written differently

for (int i=0; i<10; i++) { a[I]++;


Local Variable Declarations

! We know how to do these:<type> name;<type> name = <expr>

! But for how long is a name ‘valid’?! The part of the code where a name can be

referenced is called the name’s scope.


Scope

{int a = 10;int b = 1;while (a > 0){ int c = b * 2; b = c;

}}Scope of a in red


}}


}}

Scope of c in redScope of b in red


! The scope of a local variable is the restof the closest enclosing block!! { } creates a block! ….. Others to come later


If-statements

! Used for flow control (decisions)! Two general forms:

if (<bexpr>)<then-stmt>

…

if (<bexpr>)<then-stmt>

else<then-stmt>

If <bexpr> evaluates to true,then the <then-stmt> is executed,If <bexpr> evaluates to false,execution continues at …

If <bexpr> evaluates to true,then the <then-stmt> is executed.If <bexpr> evaluates to false,then the <else-stmt> is executed


If-statements

! Be careful about nested if-statements:if (i==j) if (j==k) System.out.println(“i = k”);else

System.out.println(“i != j”);

! An else always matches the inner-mostif. To make sure you get it right use { }


If-statements

if (i==j) { if (j==k) { System.out.println(“i = k”);}

} else { System.out.println(“i != j”);}

Technically { } and { } are not needed.


If-statements

! If-statements can alsobe chained:if (i == 2) { b = 3;} else if (i == 3) {b = 5;

} else if (i == 4) {b = 7;

} else {b = 9;

}

if (i == 2) b = 3;else if (i == 3)b = 5;

else if (i == 4)b = 7;

elseb = 9;

or without { }


Switch-statement

! A multi-case replacement for a lot of if-statementsif (n == 1) a = 2;else if (n == 2) a = 4;else if (n == 3) a = 6;else if (n == 4 || n == 5) a = 7;else a = 9;


Switch-statement

! A multi-case replacement for a lot of if-statementsif (n == 1) a = 2;else if (n == 2) a = 4;else if (n == 3) a = 6;else if (n == 4 || n == 5) a = 7;else a = 9;

switch (n) { case 1: a = 2; break; case 2: a = 4; break; case 3: a = 6; break; case 4: case 5: a = 7; break; default: a = 9; break;}

Same as


Switch-statement

! A few things are important to rememberabout switch-statements.! The default label is optional (just likeelse is!)

! The break statement for each case isoptional (but almost always needed)! When hitting a break (!) in a switch statement,

control jumps to the end of it (I.e., the } )


Switch-statement

! Consider this:a = b = c = 0;switch (n) { case 1: a = 1;

case 2: case 3: b = 1;default: c = 2;


Switch-statement



}

4321

cban


Switch-statement



}

432

1111cban


Switch-statement



}

43

11021111cban


Switch-statement



}

4110311021111cban


Switch-statement



}

1004110311021111cban


While-statement

! Simple looping mechanismwhile (<bexpr>)<statement>

! While the <bexpr> is true, execute<statement> and go back andevaluate the <bexpr>


While-statement

! Remember, if more than one statementis to be executed, use { }:

b = 1; b = 1;while (a > 0) { while (a > 0)

a = a - 1; vs. a = a - 1;b = b * 2; b = b * 2;

}


While-statement

! Remember, if more than one statementis to be executed, use { }:

b = 1; b = 1;while (a > 0) { while (a > 0)

a = a - 1; vs. a = a - 1;b = b * 2; b = b * 2;

}a = 0 and b = 2a a = 0 and b = 2


Do Statement

! Almost like a while:do statementwhile ( expression );

! statement always executes at leastonce.


While vs do

a = 100; a = 100;b = 1; b = 1;while (a > 0) { do {

a = a - 1; a = a - 1;b = b * 2; b = b * 2;

} }

! Are these two pieces of the same?


While vs do

while (expression) do statement statement

while (expression)

! Are these two pieces of the same?! Always???


While vs do

b = 0; b = 0;while (a > 0) { do {

b = b + 2; b = b + 2;a = a - 1; a = a - 1;

} } while (a > 0)

! For which values of a do they produce the sameresult?


Do statement

! Can we make a do loop work like a whileloop?while (expression) statement

if (expression) do statement while (expression)


Do statement

! Can we make a do loop work like a whileloop?while (expression) statement

if (expression) do statement while (expression)


For statement

! One last loop mechanism: the for loopfor (initialize; test; update) statement

! initialize is executed ONCE! test is executed BEFORE the statement; the

statement is executed for as long as test istrue

! update is executed after the statement


For statement

for (initialize; test; update) statement

! is equivalent toinitializetest; statement; update; test; statement; update;…test; statement; update; test;


For statement

int result = 0; for (int i=1; i<=100; i++)

result = result + i;

! Sums up the first 100 numbers.! Note:

! You may declare variables in the initializer.! The scope of these variables is the body of the loop.! More than one variable can be initialized and more than

one variable can be updated.! If declaring one or more variable in the initializer nothing

else can go there!


For statement

int result, count, i; for (result=0, count=0, i=0; i<=100; i=i+2, count++)

result = result + i;

! For loops can be written using while loopseasily:

initialize;while (test) { statement;

update;}


For/in statement

! Special type of for loop:for (declaration : expression) statement

! Often used to ‘iterate’ through acollection/arrayint[] primes = new int[] {2,3,5,7,9,11,13,17,19,23,29}for (int n: primes) System.out.println(n);


For/in statement

int[] primes = new int[] {2,3,5,7,9,11,13,17,19,23,29}for (int n: primes) System.out.println(n);

! The base type of the iterating variablemust match that of the collection.! More on arrays soon!


Skipping for now

! Break! Continue! Return


Skipping completely

! Synchronized! Throw/exceptions! Assert


Methods

! A method is a way to abstract awaycomputation.! Just like math: f(x,y) = x*x + y*y;

! It has formal paramters (x,y)! It has a (return) type: what ever type x*x + y*y is.! It can be applied to many different pairs of actual

parameters: f(6,7), f(8,9), f(g(5),5),….


Methods

! Simple example:public int f(int x, int y) { int result; result = x*x + y*y; return result;}

! public is a modifier (more about that later)! int is the type that the method returns! f is the name, int x, int y the formal

paramters.


Methods

! Simple example:public int f(int x, int y) { int result; result = x*x + y*y; return result;}

int a;a = f(5,7);

! 5 and 7 are the actual parameters.


Scoping in Methods

! Methods have the same scoping rules aswhat we have used until now except forthe parameters! The scope of a parameter is the entire body


Call by value

! A method in Java uses call by value callsemantics:void f(int a) { a = 10000;}

int a = 0;f(a);// value of a?

! The variable a is not passed to f, its VALUE is.


Methods

! When calling a method, the order of the actualparameters must match the order of the formalparameters.

! The same number of actual parameters asformals must be passed to a method.

! If a method does no return anything, its type isvoid.

! Return value may be ignored.


Arrays

! An array is a collection of values all ofthe same type (more or less!)! Either all of a primitive type! Or all of a reference type (more about that

later)! Not a primitive type, but an object type.


Arrays

! Arrays are allocated dynamically! Cannot be allocated statically on the stack,

but must be on the heap.

int[] intArray;

declares a variable that can hold areference to an array of integers.


No Static Arrays

! We cannot declare static arrays:

int[3] myLittleArray;or

int myBigArray[1000];

are both illegal.


Creating an array

! All arrays must be allocated/createdbefore they can be used.! Unlike other primitives which are just there

when declaredint a; // you can assign to a

//and use it as an intint[] as; // you cannot use as until // it has been given a

// value


Creating an Array

! Let us create an array of 100 integers:

int[] myInts;myInts = new int[100];

Or

int[] myInts = new int[100];


Indexing an array

! An array is indexed by an integer valuestarting at 0. The first element is at index0, the second at index 1 etc.

int[] myInts = new int[100];// index 0..99

for (int i=0; i<100; i++) myInts[i] = i;


Indexing an array

! The first index of an array is index 0! The last is l-1 where l is the length of

the array.! The length can be obtained like this:

int len = myInts.length;


! We could redo the code from 2 slidesago:

int[] myInts = new int[100]; // index 0..99for (int i=0; i<myInts.length; i++) myInts[i] = i;


Initializing arrays

! We can write code to initialize an arraylike on the previous slide.

! Or we can do it when the array iscreated:int[] myInts = new int[]{0, 1, 2, 3, 4};

! When initializing an array like this, nosize is given. I.e., this is illegal: int[] myInts = new int[5]{0, 1, 2, 3, 4};


Call by reference vs call by value

! Consider the following code:public static void foo(int a) { a = 900;

}

public static void main(String[] args) { int a = 0; foo(a); int b=0; foo(b);

}

! What is the value of a and b after the call to foo?




}

public static void main(String[] args) { int a = 0; foo(a); // the value of a is still 0 int b=0; foo(b); // the value of b is still 0

}

! Remember, copies of the values of a and b are transferred tofoo, and the a parameter belongs to foo, and not main!

CALL BY VALUE:COPIES of the

ACTUAL parametersare passed to the

method. Any changesmade to the parameterseffect only the FORMALparameters inside the

method, not the ACTUALparameters passed.




}

public static void main(String[] args) { int a = 0; foo(a); // the value of a is still 0 int b=0; foo(b); // the value of b is still 0

}

! Remember, copies of the values of a and b are transferred tofoo, and the a parameter belongs to foo, and not main!

CALL BY VALUE:COPIES of the

ACTUAL parametersare passed to the

method. Any changesmade to the parameterseffect only the FORMALparameters inside the

method, not the ACTUALparameters passed.

CALL BY REFERENCE:Changes made to

the FORMAL parametersWILL effect the ACTUAL

parameters.



! All primitives in Java are passed by value (call-by-value)

! Arrays are passed by reference (call-by-reference)! Any chances made in a method to an array

parameter will be made to the actual parameter


Call by reference

public class ArrayTest {

public static void fill(int[] array) { for (int i=0;i<array.length;i++) array[i]=i; // <-- will change intArray }

public static void main(String[] args) { int[] intArray = new int[100]; fill(intArray);

for(int i=0;i<100;i++) System.out.println(intArray[i]); }}


Aliasing


public static int[] fill(int[] array) { for (int i=0;i<array.length;i++) array[i]=i;

return array; }

public static void main(String[] args) { int[] intArray = new int[100];

int[] newIntArray; newIntArray = fill(intArray);

for(int i=0;i<100;i++) { System.out.println(intArray[i]);

System.out.println(newIntArray[i]); } }}


Aliasing



return array; }

public static void main(String[] args) { int[] intArray = new int[100];

int[] newIntArray; newIntArray = fill(intArray);


System.out.println(newIntArray[i]); } }}

intArray and newIntArraywill contain the same data.NOT copies of the samedata, but THE SAMEdata


Aliasingpublic class ArrayTest {


return array; }

public static void main(String[] args) { int[] intArray = new int[100] int[] newIntArray; newIntArray = fill(intArray);


System.out.println(newIntArray[i]); }}

The integer array is onlyallocated once; all other‘copies’ of it are just copiesof ‘arrows’ pointing to the same set of integers.

This is called aliasingWhen a method is invoked with an array parameter it is passed by reference


Multi-dimensional Arrays

! Java supports multi-dimensional arrays:int[][] twoDimArray;twoDimArray = new int[10][10];

! This creates a 2D array indexed like this:twoDimArray[0][0] = 100;twoDimArray[9][9] = 900;

! Java allows ‘ragged arrays’ (wherehigher dimensions’ entries aren’t thesame length.


Ragged Arrays

int[][] bs = new int[2][];bs[0] = new int[5];bs[1] = new int[]{1,2,3};

! bs is 2 dimensional

! bs[0] is 1 dimensional of type int[]! bs[1] is 1 dimensional of type int[]

bs[0]

bs[1]

bs[0][0]Val: ?

bs[0][1]Val: ?

bs[0][2]Val: ?

bs[0][3]Val: ?

bs[1][0]Val: 1

bs[0][4]Val: ?

bs[1][1]Val: 2

bs[1][2]Val: 3


Multi-dimensional arrays

! You may leave the allocation of a higherdimension to later (last slide), but youmay not leave lower ones:

int[][] bs = new[][2]; // illegal


Multi-dimensional arrayconstants

! We can write constants of multi-dimensional type as well:

int[][] myInts = new int[][]{ {1,2}, {3,4,5,6},

{7,8,9} };


Copying arrays

! As we have seen using = on arrayvariables just copies a ‘pointer’

! If we want to copy an entire array we caneither use a for loop and copy theelements by hand, or use .clone()int[] a = new int[]{1,2,3};int[] b = a.clone();a[1] = 900; // a = {1,900,3}, b={1,2,3}


Copying array

int[][] bs = new int[2][]; bs[0] = new int[]{100,200,300}; bs[1] = new int[]{1,2,3}; int[][] q = bs.clone(); bs[0][1] = 999;

System.out.println(q[0][1]);

What is printed?


Copying array

int[][] bs = new int[2][]; bs[0] = new int[]{100,200,300}; bs[1] = new int[]{1,2,3}; int[][] q = bs.clone(); bs[0][1] = 999;

System.out.println(q[0][1]);

What is printed? 999

.clone() is a SHALLOWcopy - it copies only the TOP level. If an arraycontains entries that arereference, then they arecopied as REFERENCESnot as VALUES.


Traversing an array

! We are familiar with this:int sum = 0;for(int i=0; i<myArray.lengh; i++) sum = sum + myArray[i]

! We can use a different for-loop int sum = 0;

for (int e : myArray) sum = sum + e;

Documents

Introduction - University of Nevada, Las Vegas