What is special about the String class in Java (immutable? + operator overloading? new String("abc") creates several objects?)

1. Why is String immutable?

A brief description: This can be seen in the JDK source code. String is essentially a char[] array. And with final modification, in JDK1.7, there are mainly two member variables of String.

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {

     /** String本质是个char数组. 而且用final关键字修饰.*/
    /** The value is used for character storage. */
    private final char value[];

    /** Cache the hash code for the string */
    private int hash; // Default to 0
    ....
    ....

Final modified char[] array, the reference is immutable , although the reference is immutable. That doesn't stop the fact that Array arrays are mutable. The data structure of Array is shown in the figure below
write picture description here

final int[] value={1,2,3}
int[] another={4,5,6};
value=another; //编译器报错，final不可变

The value is decorated with final, and the compiler does not allow us to point the value to another address in the heap area. But if I do it directly on the array elements, it will be done in minutes.

final int[] value={1,2,3};
value[2]=100; //这时候数组里已经是{1,2,100}

However, the char array is defined as private in the source code, and the private members cannot be accessed and changed externally. So in summary String is immutable.

For a detailed analysis, see
① Why is String in Java immutable? – String source code analysis
② How does Zhihu understand the immutability of String type values?

What is special about the String class in Java (immutable? + operator overloading? new String("abc") creates several objects?)

1. Why is String immutable?

2. How many objects are created by String s=new String("abc")?

3. String class and constant pool

4. Is the concatenation of String + operator overloading?

5. What is the difference between String, StringBuilder and StringBuffer?

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