字符串String Stringbuilder StringBuffer

一、 String Stringbuilder Stringbuffer简介

1.1 String

字符串常量，字符串长度不可变（immutable）
String 类源码（截取常用方法和属性）

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
   
   
    private final char value[];
    private int hash; // Default to 0
    
    public String() {
   
   
        this.value = "".value;
    }
    
    public String(char value[]) {
   
   
        this.value = Arrays.copyOf(value, value.length);
    }

    public String(StringBuffer buffer) {
   
   
        synchronized(buffer) {
   
   
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }
   
    public String(StringBuilder builder) {
   
   
        this.value = Arrays.copyOf(builder.getValue(), builder.length());
    }
    
    public int length() {
   
   
        return value.length;
    }

    public boolean isEmpty() {
   
   
        return value.length == 0;
    }
    
    public char charAt(int index) {
   
   
        if ((index < 0) || (index >= value.length)) {
   
   
            throw new StringIndexOutOfBoundsException(index);
        }
        return value[index];
    }
    
    public boolean equals(Object anObject) {
   
   
        if (this == anObject) {
   
   
            return true;
        }
        if (anObject instanceof String) {
   
   
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {