1.构造方法
public String()
public String(byte[] bytes)
public String(byte[] bytes, int index, int length)
public String(char[] value)
public String(char value, int index, int count)
2.判断功能
boolean equals(Object obj)
boolean equalsIgnoreCase(String str)
boolean contains(String str)
boolean startsWith(String str)
boolean endsWith(String str)
boolean isEmpty()
3.获取功能
public int length()
public char charAt(int index)
public int indexOf(int ch)
public int indexOf(int ch, int fromIndex)
public int indexOf(String str)
public int indexOf(String str, int fromIndex)
public String substring(int start)
public String substring(int start, int end)
4.转换功能
public String(byte[] bytes)
public String(byte[] bytes, int index, int length)
public byte[] getBytes()
public static String valueOf(char[] chs)
public char[] toCharArray()
public static String valueOf(int i) //任意类型转字符串，字节数组除外
public String toUpperCase()
public String toLowCase()
public String concat(String str)
5.其他功能
String replace(char old, char new)
String replace(String old, String new)
length()结合charAt()
字符串转为字符数组toCharArray()
