String相关类

String

• 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
  • 内置final修饰的字符数组，String主要的行为就是管理内置字符数组
  • String是一个Final类，代表不可继承更改的对象
  • 新建String内存分配
    • 字符常量在方法区的常量区
      • 常量相加，在常量区
    • new关键字创建的皆在gc区
      • new创建的String类型和任意类型做拼接皆在gc区
    • intern();把string压入常量池，若有则不会添加。
  • 类声明：

  //源码111行112行
  public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
  

  • 主要属性：

  //源码114行
  private final char value[];
  

  • 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.

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StringBuffer

• 内容可变字符串序列
• 很多方法和String相同，区别是长度可变
• 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
• 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

StringBuilder类

• StringBuffer的增强，比stringbuffer效率高，完全兼容StringBuffer（使用一致），线程不安全。