字符串类

字符串相关类

String类，构造字符串对象

常量对象，使用双引号括起来的字符序列，字符串一旦创建，不能改变
字符串的字符使用Unicode字符编码，一个字符占两个字节
可以简单理解为是一个字符数组
String是一个Final类，代表不可变的字符序列
new和+等产生新对象的在gc区，
常量相加，在常量区
字符常量在方法区的常量区
intern();把string压入常量池，若有则不会添加。
总
- 字面量在常量区
- 对各常量相加结果总是常量区
- 多个字符串相加，里面若有一个变量则在gc区
- new创建对象，在gc区
原理：
- 字符串
- 常用方法
  - int length()
    - Returns the length of this string.
  - char charsAt(int index)
    - Returns the char value at the specified(指定的) index.
  - char[] toCharArray()
    - Converts(转换)this string to a new character array.
  - boolean equals(Object anObject)
    - Compares this string to the specified(指定的) object.
  - int compareTo(String anotherString)
    - Compares two strings lexicographically(字典序).
  - int indexOf(String str) 返回str在字符串中出现的下标
    - Returns the index within this string of the first occurrence of the specified(指定) substring(子串).
  - int indexOf(String str, int fromIndex)
    - Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
  - int lastIndexOf(String str)
    - Returns the index within this string of the last occurrence of the specified substring.
  - int lastIndexOf(String str, int fromIndex)
    - Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
  - boolean startsWith(String prefix, int toffset)
    - Tests if the substring of this string beginning at the specified index starts with the specified prefix.
  - boolean endsWith(String suffix)
    - Tests if this string ends with the specified suffix.
  - String substring(int beginIndex, int endIndex) 截取子串
    - Returns a string that is a substring of this string.
  - String substring(int beginIndex) 截取子串
    - Returns a string that is a substring of this string.
  - String replace(char oldChar, char newChar) 替换
    - Returns a string resulting from replacing all occurrences of oldChar in this string with newChar.
  - String replaceAll(String regex, String replacement) 替换字符串
    - Replaces each substring of this string that matches the given regular expression with the given replacement.
  - String trim() 修剪首尾空白字符（小于等于32的）
    - Returns a string whose value is this string, with any leading(首) and trailing(尾) whitespace(空白字符) removed.
  - String toUpperCase() 转大写
    - Converts all of the characters in this String to upper case using the rules of the default locale.
  - String toLowerCase() 转小写
    - Converts all of the characters in this String to lower case using the rules of the default locale.
  - String[] split(String regex) 切割
    - Splits(切割) this string around matches of the given regular(正则表达式) expression.
  - boolean equalsIgnoreCase(String anotherString) 比较两个字符串是否一致，忽略大小写
    - Compares this String to another String, ignoring(忽略) case considerations.

//1.8.0_181版 jdk中String类源码第114行
private final char value[];     

//String 部分源码
/**
     * Converts this string to a new character array.
     *
     * @return  a newly allocated character array whose length is the length
     *          of this string and whose contents are initialized to contain
     *          the character sequence represented by this string.
     */
    public char[] toCharArray() {
        // Cannot use Arrays.copyOf because of class initialization order issues
        char result[] = new char[value.length];
        System.arraycopy(value, 0, result, 0, value.length);
        return result;
    }
/**
     * Compares this string to the specified object.  The result is {@code
     * true} if and only if the argument is not {@code null} and is a {@code
     * String} object that represents the same sequence of characters as this
     * object.
     *
     * @param  anObject
     *         The object to compare this {@code String} against
     *
     * @return  {@code true} if the given object represents a {@code String}
     *          equivalent to this string, {@code false} otherwise
     *
     * @see  #compareTo(String)
     * @see  #equalsIgnoreCase(String)
     */
    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) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

StringBuffer类

内容可变字符串序列
很多方法和String相同，就是长度可变
容器，装载类型为char
构造器
- StringBuffer() 无参
  - Constructs a string buffer with no characters in it and an initial capacity of 16 characters.
- StringBuffer(String str)
常用方法
- append() 多态，方法很多，可认为时可添加任意类型，直接修改StringBuffer本身
  - 返回值为本身，可以连用append来，无参构造预置数组为16；
- insert(int, Objects) 插入，任意数据类型
- reverse() 反转
- delete(int,int) 删除
- setCharAt(int,Objects) 修改

StringBuilder类

StringBuffer比较老的版本特点，线程安全，速度慢
StringBuilder比较新的版本特点，速度快，性能好
StringBuffer的增强，比stringbuffer效率高，使用一样。