java - 数组和字符串充当对象

Question

class Effect 
{
public static void main(String [] args) 
{
    Effect e=new Effect();
    e.method();
}
void method()
{
    long [] a1 = {3,4,5};
    long [] a2 = doArray(a1);
    //expected output
    System.out.println("After Calling doArray "+a1[1] +" "+ a2[1]);

    String s1 = "Hello";
    String s2 = doString(s1);
    //expected output s1=HelloJava s2=World  like earlier arrays
    System.out.println("After Calling doString "+s1 + " " + s2);
}

long [] doArray(long [] a3) 
{
    a3[1] = 7;
    System.out.println("In doArray "+ a3[0]+" "+a3[1]+" "+a3[2]);
    return a3;
}
 String doString(String s1) 
{
    s1 = s1 + "Java";
    System.out.println("In doString "+ s1);
    return "World";
}

}

输出

In doArray 3 7 5
After Calling doArray 7 7
In doString HelloJava 
After Calling doString Hello World

我的预期输出:

After Calling doString HelloJava World

• 所有数组引用 a1,a2,a3 指向同一个。所以，修改后的数据适用于所有
• 但如果是 String，即使我们修改数据，它也会给出旧值

请解释一下？

Answer 1

在 java 中，String 对象是不可变的。

The String class is immutable, so that once it is created a String object cannot be changed. The String class has a number of methods, some of which will be discussed below, that appear to modify strings. Since strings are immutable, what these methods really do is create and return a new string that contains the result of the operation.

Documentation

代码说明 -

String doString(String s1) {
    s1 = s1 + "Java";
    ...
}

此处s1+"java" 将在字符串池 中创建一个新对象。并且从 method 函数引用 s1 仍将引用旧的 String 对象。