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What is the reason why the String class in Java is immutable? I believe many inexperienced people don't know what to do about it. Therefore, this paper summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

Why are String classes designed to be immutable?

String is designed to be immutable, mainly in terms of performance and security.

1. The need for constant pool

It's easy to understand that the string constant pool in Java exists for performance optimization.

String constant pool (String pool) is a special storage area in Java heap memory. When creating a String object, if the string already exists in the constant pool, a new object is not created, but an existing object is referenced directly. Doing so reduces the memory overhead of JVM and improves efficiency.

For example, references S1 and S2 both point to the same object "abc" of the constant pool. If String is a mutable class, referring to S1's modification of the String object will directly cause the reference S2 to get the wrong value.

String S1 = "abc"

String S2 = "abc"

So, if the string is mutable, then the constant pool does not make sense.

2. Need for hashcode caching

Because the string is immutable, the hashcode is cached when it is created and does not need to be recalculated. This makes the string very suitable as a key in HashMap, and the efficiency is greatly improved.

3. Multithread safety

In multithreading, the value of a mutable object is likely to be changed by other threads, resulting in unexpected results. The immutable String can be freely shared among multiple threads without the need for synchronous processing.

How is the String class immutable? 1. Private member variables

The interior of String is simple, with two private member variables

/ * The value is used for character storage. , /

Private final char value []

/ * * Cache the hash code for the string * /

Private int hash; / / Default to 0

There is no external way to modify these two properties.

2. The method of Public is to copy a copy of data.

String has many public methods, each of which creates a new String object, such as the substring method:

Public String substring (int beginIndex) {

If (beginIndex < 0) {

Throw new StringIndexOutOfBoundsException (beginIndex)

}

Int subLen = value.length-beginIndex

If (subLen < 0) {

Throw new StringIndexOutOfBoundsException (subLen)

}

Return (beginIndex = = 0)? This: new String (value, beginIndex, subLen)

}

3. String belongs to final

String is decorated by final, so we can't inherit String, so we can't override some methods through inheritance.

Public final class String

Implements java.io.Serializable, Comparable, CharSequence {

}

4. Deep copy of constructor

When the variable array value [] is passed in, copy is performed instead of copying value [] directly to the internal variable.

Public String (char value []) {

This.value = Arrays.copyOf (value, value.length)

}

From the way String classes are designed, we can summarize the ways to implement immutable classes:

Declare class itself as final so that others can't get around the limit by extending it. Define all member variables as private and final, and do not implement the setter method. When constructing an object, member variables are initialized with a deep copy rather than a direct assignment, which is a defense because you are not sure that the input object will not be modified by others. If you do need the getter method, or any other method that may return an internal state, use the copy-on-write principle to create a private copy. After reading the above, have you mastered the method of why the String class is immutable in Java? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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