Java array expansion code case sharing

This article mainly introduces the expansion of Java arrays. The implementation code is very simple and has certain reference value.

In the process of writing programs, we often encounter situations where the array space is not enough. For example, I have initialized an array int []a = {1,2,3,4,5,6,7 ,8,9,10} ;At this time, I want to insert an element into the array at index 3. What should I do? It's too difficult to implement in C language. You need to call the memcpy function one by one, but in Java, it doesn't have to be so troublesome. There is a way to expand the capacity of an array, which can be easily implemented. Let’s take a look at the code:

public class HelloWorld {
	public static void main(String[] args){
	//	Scanner s = new Scanner(System.in);
	//	System.out.println("请输入一个数字");
	//	int num = s.nextInt();
		//这个数组下标只能到9
		int []a = {1,2,3,4,5,6,7,8,9,10};
		//先扩容
		int []b = Getarray(a,3,100);
		Demoe.PrintArray(b);
	}
	//案例：有一个数组已经初始化完毕，现向其下标3插入一个元素100
	public static int[] Getarray(int []a , int index , int num){
		int src_array_length = a.length ; 
		int []b = new int[src_array_length+1] ;
		for(int i = 0 ; i < a.length ; i++){
			b[i] = a[i] ; 
		}
		a = b ; //如果还想使用a,使得a指向b
		//接下来从最后一个元素开始，将最后一个元素的前面一个元素拷贝到最后一个
		//以此类推
		for(int i = a.length - 1 ; i > index ; i--){
			a[i] = a[i-1];
		}
		//别忘了，将要插入的元素插入到对应的数组坐标
		a[index] = num ;
		return a ;
	}
}

You can also use library functions in java to implement

import java.util.Arrays;
public class HelloWorld {
	public static void main(String[] args){
	//	Scanner s = new Scanner(System.in);
	//	System.out.println("请输入一个数字");
	//	int num = s.nextInt();
	int []a = {1,2,3,4,5};
	int []b = new int[a.length] ;
		//1、源数组
		//2、源数组的哪个位置开始
		//3、目标数组
		//4、目标数组的哪个位置开始存放
		//5、拷贝几个
		//System.arraycopy(a, 0, b, 0, a.length);
		//Demoe.PrintArray(b);
		//扩容--->扩容了一倍
		//a = Arrays.copyOf(a, 2*a.length) ;
		//Demoe.PrintArray(a);
		a = insertArray(a , 3 , 100) ;
		//打印数组
		System.out.println(Arrays.toString(a));
	}
	//写一个函数，向整数数组的任意pos位置插入一个元素value
	public static int[] insertArray(int []a , int pos , int value){
		if(pos < 0 || pos > a.length + 1) //索引下标不对，直接返回源数组 
			return a ;
		//放入一个元素，先扩容，后复制
		a = Arrays.copyOf(a, a.length+1) ;
		for(int i = a.length - 1 ; i > pos ; i--){
			a[i] = a[i-1] ; //后移动
		}
		a[pos] = value ;
		return a ;
	}
}

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