Java怎么用for循环给数组赋值？

[gjmding]

package les9;

import java.util.Scanner;

public class Book {

	/**
	 * @param args
	 */
	private static int bookNum;
	private static double price;
	public Book(int bookNum,double price){
		this.bookNum=bookNum;
		this.price=price;
	}
	
	public static double MaxPrice(Book[] b){
		double MaxPrice=Double.MIN_VALUE;
		for(int i=0;i<b.length;i++){
			if(b[i].bookNum*b[i].price>MaxPrice){
				MaxPrice=b[i].bookNum*b[i].price;
			}
		}
		return MaxPrice;
	}
	
	public static double MinPrice(Book[] b){
		double MinPrice=Double.MAX_VALUE;
		for(int i=0;i<b.length;i++){
			if(b[i].bookNum*b[i].price<MinPrice){
				MinPrice=b[i].bookNum*b[i].price;
			}
		}
		return MinPrice;
	}
	
	public static void show() {
		System.out.println("数量:"+bookNum+"\t单价:"+price);
	}
	
	public static void showAll(Book[] b) {
		for(int i=0;i<b.length;i++) {
			System.out.println("数量:"+b[i].bookNum+"\t单价:"+b[i].price);
		}
	}
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Book b[]=new Book[2];
		Scanner sc=new Scanner(System.in);
		for(int i=0;i<b.length;i++){
			System.out.println("请输入第"+(i+1)+"本书的单价:");
			int num=sc.nextInt();
			//System.out.println(num);
			//System.out.println(b[i].bookNum);
			System.out.println("请输入第"+(i+1)+"本书的数量:");
			double pri=sc.nextDouble();
			//System.out.println(b[i].price);
			//System.out.println("该书的总价为:"+b[i].bookNum*b[i].price);
			b[i]=new Book(num,pri);
			Book.show();
		}
		showAll(b);
		System.out.println("最低价为:"+Book.MinPrice(b));
		System.out.println("最高价为:"+Book.MaxPrice(b));
	}

}


---------------------------------------------------------------------------------------------
代码如图，主要的问题是，如何在new一个对象的前提下，对数组进行循环赋值，并且能够输出前一轮循环赋值的数据，麻烦各位指点一下

[abcdefghiijklmnopqrs]

public class Book {
	 
    /**
     * @param args
     */
    private int bookNum;
    private double price;
    public Book(int bookNum,double price){
        this.bookNum=bookNum;
        this.price=price;
    }
     
    public static double MaxPrice(Book[] b){
        double MaxPrice=Double.MIN_VALUE;
        for(int i=0;i<b.length;i++){
            if(b[i].bookNum*b[i].price>MaxPrice){
                MaxPrice=b[i].bookNum*b[i].price;
            }
        }
        return MaxPrice;
    }
     
    public static double MinPrice(Book[] b){
        double MinPrice=Double.MAX_VALUE;
        for(int i=0;i<b.length;i++){
            if(b[i].bookNum*b[i].price<MinPrice){
                MinPrice=b[i].bookNum*b[i].price;
            }
        }
        return MinPrice;
    }
     
    public void show() {
        System.out.println("数量:"+bookNum+"\t单价:"+price);
    }
     
    public static void showAll(Book[] b) {
        for(int i=0;i<b.length;i++) {
            System.out.println("数量:"+b[i].bookNum+"\t单价:"+b[i].price);
        }
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Book b[]=new Book[2];
        Scanner sc=new Scanner(System.in);
        for(int i=0;i<b.length;i++){
            System.out.println("请输入第"+(i+1)+"本书的单价:");
            int num=sc.nextInt();
            //System.out.println(num);
            //System.out.println(b[i].bookNum);
            System.out.println("请输入第"+(i+1)+"本书的数量:");
            double pri=sc.nextDouble();
            //System.out.println(b[i].price);
            //System.out.println("该书的总价为:"+b[i].bookNum*b[i].price);
            b[i]=new Book(num,pri);
            b[i].show();
        }
        showAll(b);
        System.out.println("最低价为:"+Book.MinPrice(b));
        System.out.println("最高价为:"+Book.MaxPrice(b));
    }
 
}

不知道你说的是什么意思

[gjmding]

利用for循环给Book的数组赋值

[clj23]

public class Book {

/**
* @param args
*/
private int bookNum;
private double price;
public Book(int bookNum,double price){
this.bookNum=bookNum;
this.price=price;
}

public static double MaxPrice(Book[] b){
double MaxPrice=Double.MIN_VALUE;
for(int i=0;i<b.length;i++){
if(b[i].bookNum*b[i].price>MaxPrice){
MaxPrice=b[i].bookNum*b[i].price;
}
}
return MaxPrice;
}

public static double MinPrice(Book[] b){
double MinPrice=Double.MAX_VALUE;
for(int i=0;i<b.length;i++){
if(b[i].bookNum*b[i].price<MinPrice){
MinPrice=b[i].bookNum*b[i].price;
}
}
return MinPrice;
}

// public static void show() {
// System.out.println("数量:"+bookNum+"\t单价:"+price);
// }

public static void showAll(Book[] b) {
for(int i=0;i<b.length;i++) {
System.out.println("数量:"+b[i].bookNum+"\t单价:"+b[i].price);
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner sc=new Scanner(System.in);
while(true) {
Book b[]=new Book[2];
for (int i = 0; i<b.length; i++) {
System.out.println("请输入第" + (i + 1) + "本书的单价:");
int num = sc.nextInt();
//System.out.println(num);
//System.out.println(b[i].bookNum);
System.out.println("请输入第" + (i + 1) + "本书的数量:");
double pri = sc.nextDouble();
//System.out.println(b[i].price);
//System.out.println("该书的总价为:"+b[i].bookNum*b[i].price);
b[i]=new Book(num,pri);
// Book.show();

}
showAll(b);
System.out.println("最低价为:" + Book.MinPrice(b));
System.out.println("最高价为:" + Book.MaxPrice(b));
}
}

}

[gjmding]

问题已经解决了。。。但是还是没能明白为什么数组的元素为空就一定会报错？比如说定义了10个对象，但是我只给4个对象赋值了，然后就算设置if（数组元素！=null）也解决不了Java报错NullPointerException，实在不知道问题出在哪里，是因为Java的数组一定要有值吗？