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1.1 Programming Model

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读完本文你会了解到：

  • Java有关数组的基础知识
  • 静态方法的深入理解-见参考
  • 练习一下二分查找

Primitive types:

data type representation
byte 8-bit
char 16-bit
short 16-bit
int 32-bit
float 32-bit
long 64-bit
double 64-bit

声明语句：声明语句将一个变量名和一个类型在编译时关联起来。Java是一种强类型的语言，因为Java编译器会检查类型的一致性。

循环语句：

  • break语句，立即从循环中退出
  • continue语句，立即开始下一循环

Array

Definition:An arroy stores a sequence of values that are all of the same type.if we have N values,we can use the notation a[i] to refer ith value of i from 0 to N-1.

Making an array involves three distinct steps:

  • Declare the array name and type
  • Creat the array
  • Initialize the array values

Example:

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double[] a;               //declaration
a = new double[N];        //creation
for(int i = 0; i<N ; i++) //initialization
    a[i] = 0.0;
  • Using:Java does automatic bounds checking-if you access an array with an illegal index your program will terminate with an ArrayIndexOutOfBoundsException
  • Alising(混淆):An array name refers to the whole array-if wo assign one array name to another,then both refer to the same array,as illustrate in the following code fragment.
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int [] a = new int[N];
...
a[i] = 1234;
...
int [] b = a;
...
b[i] = 5678;   //a[i] = 5678.

有关数组的典型静态方法实现：

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//颠倒数组的元素
int N = a.length;
for (int i=0; i < N/2; i++) {
	double temp = a[i];
	a[i] = a[N-1-i];
	a[N-1-i] = temp;
}
//矩阵相乘
int N = a.length;
double[][] c = new double[N][N];
for (int i=0;i < N; i++)
	for (int j=0; j < N; j++){
		for (int k=0;k < N; k++)
			c[i][j] += a[i][k]*a[k][j];
	}

Static method

  • Defining a static method:A method encapsulates a computation that is defined as a statements. A method takes arguments and computes a return value of some data type or causes a side effect.
  • Java method have features:
    • Arguments are passd by value.
    • Method names can be overloaded.
    • A method has a signal return but may have multiple return statements.
    • A method can have side effects.
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public class BinarySearch{
	//前提：该数组必须是有序的
	public static int rank(int key, int[] a){
		int lo = 0;
		int hi = a.length - 1;
		while(lo <= hi){
			int mid = lo + (hi - lo)/2;
			if (key < a[mid]) hi = mid - 1;
			if (key > a[mid]) lo = mid + 1;
			else return mid;
		}
		return -1;
	}
}

Web Exercises

1 Wget. Write a program Wget.java that reads in data from the URL specifiedon the command line and saves it in a file with the same name.

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public static void main(String[] args)
{
	//read in data from URL
    String URL = args[0];
    In in = new In(URL);
    String data = in.readAll();
  
    //write data to a file
    String filename = URL.substring(URL.lastIndexOf('/')+1);
    Out out = new Out(filename);
    out.println(data);
    out.close;
}

2 编写一段代码，将一个正整数N用二进制表示并转换为一个String类型的值s。

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String s = "";
for(int n = N; n > 0; n /= 2)
   s = (n % 2) + s;

同理，用m进制表示，就将2换成m进制

3 Right triangle.Write a client RightTriangle.java that draws a right triangle and a circumscribing circle.

4 Bouncing ball.Write a program BouncingBall.java that illustrates the motion of a bouncing ball

参考

static method: