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SP1 Lecture 5: 9/2/11:. Static and instance variables Class Math Random Loan calculator Local variables Array (if time permits). Static and instance variables. Static variable :belongs to its class, and it is shared by all class instances, with the same value - PowerPoint PPT Presentation
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SP1 Lecture 5: 9/2/11:
• Static and instance variables• Class Math
– Random – Loan calculator
• Local variables• Array (if time permits)
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Static and instance variables
Static variable: belongs to its class, and it is shared by all class instances, with the same value
Instance variable: a class variable without the “static” modifier, is shared by all class instances, but its values can differ in different instances
Local variable: is created within a method or instance in a { } block. Its scope is limited within the block.
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Example (1)public class TesNum { int instVar = 1; static int statVar = 10;TesNum() { System.out.println("test: " + instVar +
" and " + statVar); instVar = 7; statVar = 5; } \\ constructor
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Example(2)public static void main(String[] args) { TesNum alpha1 = new TesNum(); alpha1.instVar = 3; alpha1.statVar = 6; //syn. to: TesNum.statVar
= 6;
TesNum alpha2 = new TesNum(); System.out.println("inst: " + alpha1.instVar + "
and " + alpha2.instVar); System.out.println("stat: " + alpha1.statVar + "
and " + alpha2.statVar);//System.out.print("mix: " + instVar + " and " + statVar);
wrong
}//end of main
}//end of class
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What’s going on in TesNum
instVar statVar 1. With the class:
1 (in class) 102. At the constructor in class (virtual):
7 53. After alpha1:Constructor prints: 1 and 10
3 (within alpha1) 64. After alpha2:Constructorprints: 1 and 6
7 (within alpha2) 55. Method main prints: 3 and 7
5 and 5
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A method added:
public int SS(int a){ int b=instVar; int sum=0; if (a>b){ //swap a and b
int c=b; b=a; a=c;} for(int i=a;i<=b;i++) sum=sum+i; return sum; }// computes the sum of integers from a to b
int b1=alpha1.SS(statVar);int b2=alpha2.SS(statVar);System.out.println("sum : " + b1 + " and " +
b2);
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Sums to be printed
From alpha1: a=5, b=3The sum: 3+4+5=12, that is,
b1=12From alpha2: a=5, b=7
The sum: 5+6+7=18, that is, b2=18
The print:sum: 12 and 18
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References to object variables and methods
Examples from TesNum
alpha1.statVar
alpha2.instVar
TesNum.statVar
from TesNMod
alpha1.SS(statVar)
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Class Math (no need to import)
Math.pi =3.14…, the ratio of the
circumference to its diameter Math.abs(a) a if a >= 0, or -a if a <
0 Math.log(a) the natural logarithm (base e) of number a Math.sqrt(a) square root of number
a Math.pow(a,b) ab ; if b is an integer
then ab =aa…a (b times)
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Loan calculator I• Deposit:
– You put £1000 with annual interest rate 5%– Q: How much money it will be in 10 years?– A: Run Java loop:
• int A=1000;• for (int k=1;k<=10;k++)•A=A*1.05;•System.out.println(“In 10 years the value
is ”+A);
• Loan– You take £1000 with annual interest rate
5% for 10 years.– Q: How much money to pay monthly?
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Loan calculator II• Loan
– You take £1000 with annual interest rate 5% for 10 years.
– Q: How much money to pay monthly?– No simple arithmetic answer; powers needed
• Java computation– double a=1000.0; double air=0.05; – int period=10; – double mopay, totpay;– double mir=air/12; int
mp=period*12;//month unit
– mopay=(a*mir)/(1 – Math.pow(1/(1+mir),mp));– totpay=mopay*mp;
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Loan calculator III• public class Loan{• public static void main(String[] args) {• Loan lo = new Loan();• double mopay=lo.pay(10000, 0.05,10);• double topay=mopay*12*10;• System.out.println("Monthly payment is "+mopay);• System.out.println("Total payment is "+topay);• }
• double pay(double a, double air,int period){• double mir=air/12; int mp=period*12;//month unit• double mop=(a*mir)/(1 -
Math.pow(1/(1+mir),mp));• return mop;• }• }
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Loan calculator IV (Homework)
Modify class loan in such a way that its constructor takes from the user some or all of the details needed to do the computation:
–Loan value–Repayment period (in years)–Rate
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Math.random()pseudorandom number: double
within interval [0.0, 1.0) (zero included, unity not)
How to use it to generate a random integer between 1 and 6 (inclusive), to imitate casting a dice?
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Casting a dicedouble aa=Math.random();
//aa, a real number between 0 and 1int an= 6*aa; //a real number between 0
and 6int rand=(int) an;
// whole number between 0 and 5int randw=rand+1;
// whole number between 1 and 6 The same in one line:int randw= (int) (6*Math.random()+1);
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Casting a dice questionHow to generate a random integer
between 10 and 20 inclusive? Answer:int rdt= (int) (11*Math.random()+10);
Another possibility: using class
Randomwithimport java.util.Random
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Local variables: Definition
Local variable:
is created within a method or instance in a { } (curly brace) block.
Its scope is limited within the block.
Therefore, same name can be used in different blocks for different variables.
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Local variables(1)public class Prog03{ private static int i=3; public static void method1(){ int i=2; i+=6;
System.out.println(i); } public static int method2(int a){ a=a+3; int i=2*a-6; return i; } public static void method3(int i){ System.out.println(i+1); i=i+2; System.out.println(method2(i)); }
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Local variables(2)public static void main(String[] args){ System.out.println(i); System.out.println(i+1); method1(); i = method2(i); System.out.println(i); method3(i+1); }} //end of class
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Actual printoutWorking from main method:
Print Why 3 First line executed, static i=3
4 Second line executed, static i=3
8 Third line executed, method1 at which i=8
6 Fifth line executed with i=method2(3), that is, i=6
8 Sixth line executed, method3(9), two printings
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Array (1) Array is an indexed list of elements of the
same type; the index is supplied by default (!)
A string array nam[ ]: contains both entries and index.
String nam[] ={“John”,“Paul”,“George”,“Ringo”}; Index: 0 1 2 3
Length (the number of entries) is 4
An integer array age[ ]: int age[ ]= {23, 32, 19, 30, 25, 25, 23, 30}; Index: 0 1 2 3 4 5 6 7
Length is 8
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Array (2) Not an array: abc[ ]={8, Ringo, +} - WHY? (different types) [ ] - on the array name's right is used
to indicate arrays2. Declaring arrays Both, int ages[ ]; and int[ ] ages; is OK
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Array (3)
Initialisation of an array: either ages = new int[8]; // array with 8 zeros or ages[ ] = {23, 32, 19, 30, 25, 25, 23,
30}; //specify what is needed Simultaneously declaring & initialising (with
zeros)
int ages[] = new int[8];
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Array (4)
ages[ ] = {23, 32, 19, 30, 25, 25, 23, 30};
Accessing array elements ages [1] is 32
int i=4;int j = ages [i]; // assigning j with 25
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Work with arrays(1)Data of 5 students:
double height[ ]={1.56, 1.72, 1.80, 1.85, 1.90}; //in m
double weight[ ]={65.3,80.0,78.1,76.5,112.8}; // in kg
Problem: compute the body mass index for all the students, bmi=weight/height2
(in the US, those with bmi between 20 and 25 are considered of normal weight)
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Work with arrays(2)Loop for is natural with arrays: the index used as the
counterbmi[ ]=new double[5];for (int I = 0; I < 5; I + +)
bmi[I]=weight[I] / (height[I]height[I]); If length of student arrays is not known or is
variable, put array’s length whatever it is: bmi[ ]=new double[height.length];for (int I = 0; I < height.length; I + +)
bmi[I]=weight[I] / (height[I]height[I]);
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Work with arrays(3)
The same result with a method for the bmi:
double[ ] bmiindex(double h[ ], double w[ ]){
double in[ ];
for (int ii = 0; ii < h.length; ii = ii+1)
in[ii]=h[ii]/(w[ii]w[ii]);
return in; }
Method bmiindex is just a frame box; to make it work, one needs to put within a class this:
double[ ] bmi=bmiindex(weight, height);