源码现状
浏览源码能够发现，String被final润饰，其value变量也被final润饰，这是2件事；
首先，String类被final润饰，类不能够被继承。躲避了有人想尝试通过继承重写来毁坏String外部的数据；
其次，外部成员变量char value[] 被final润饰，value援用地址不可变；
起因可分为
平安
1.应用平安，String是最罕用的对象，不可变躲避了间接在堆中间接扭转对象内容，除非被动批改援用地址，否则咱们将String传递进任何办法中，他都不会扭转，避免一不小心就在其余函数内被更改了；
2.线程平安，并发应用String时，不须要再去思考他的线程平安问题；
性能
1.字符串缓冲池，应用享元模式，缩小String对象的产生，而享元模式的弊病就是对象的外部状态不可变；
2.hash算法，因为String初始化就不可变，所以一个String的hashcode是固定的不变的，能够只计算一次，并且String是最适宜作为hashMap中的Key；
public final class String implements java.io.Serializable, Comparable<String>, CharSequence { /** The value is used for character storage. */ private final char value[]; /** Cache the hash code for the string */ private int hash; // Default to 0 /** use serialVersionUID from JDK 1.0.2 for interoperability */ private static final long serialVersionUID = -6849794470754667710L; /** * Class String is special cased within the Serialization Stream Protocol. * * A String instance is written into an ObjectOutputStream according to * <a href="{@docRoot}/../platform/serialization/spec/output.html"> * Object Serialization Specification, Section 6.2, "Stream Elements"</a> */ private static final ObjectStreamField[] serialPersistentFields = new ObjectStreamField[0]; /** * Initializes a newly created {@code String} object so that it represents * an empty character sequence. Note that use of this constructor is * unnecessary since Strings are immutable. */ public String() { this.value = "".value; } /** * Initializes a newly created {@code String} object so that it represents * the same sequence of characters as the argument; in other words, the * newly created string is a copy of the argument string. Unless an * explicit copy of {@code original} is needed, use of this constructor is * unnecessary since Strings are immutable. * * @param original * A {@code String} */ public String(String original) { this.value = original.value; this.hash = original.hash; }}