Java笔记—String类常用方法

String类的对象创建后不能修改，用其方法“修改”的字符串其实是生成了一个新的String对象并返回，原来的String对象仍存在且未被修改

 

获取、查找方法

 public char charAt(int index) 　　// 以char返回指定索引处的字符

String str = "Change the world by program";
char result = str.charAt(2);
System.out.println(result);    //a

 

public int length()　　//获取字符串长度

String str = "Change the world by program";
System.out.println(str.length());    //27

 

public String substring(int beginIndex)　或　public String substring(int beginIndex, int endIndex)　

//返回指定索引值之间的新字符串

String str = "Change the world by program";
String str2 = str.substring(7);
String str3 = str.substring(7,16);
System.out.println(str2);    //the world by program
System.out.println(str3);    //the world

 

public int indexOf(int ch)　　//返回指定字符在此字符串中第一次出现处的索引。
public int indexOf(int ch, int fromIndex)　　//从fromIndex处开始搜索
public int indexOf(String str)　　//返回指定字符串在此字符串中第一次出现的索引，返回的是首字母位置。
public int indexOf(String str, int fromIndex)　　/从fromIndex处开始搜索
//以上方法若未能成功查找，则返回-1
另：int lastIndexOf(int ch)表示反向搜索

String str = "Change the world by program";
//正向查找
System.out.println(str.indexOf('a'));　　　　　　　　　  //2
System.out.println(str.indexOf('a',10));　　　　　　    //25
System.out.println(str.indexOf("program"));　         //20
System.out.println(str.indexOf("program",21));　　　　 //-1
//反向查找
System.out.println(str.lastIndexOf('a'));             //25  
System.out.println(str.lastIndexOf('a',10));          //2
System.out.println(str.lastIndexOf("program"));       //20
System.out.println(str.lastIndexOf("program",19));    //-1

 

public String concat(String str)　　//将指定的字符串连接到该字符串的末尾

String str = "Change the world by program";
System.out.println(str.concat(" ~tail"));
//Change the world by program ~tail
//若参数字符串长度为0，则返回原来的String对象

public String toLowerCase()　　//将字符串转化为小写
public String toUpperCase()　　//将字符串转化为大写

String str1 = "CHANGE THE WORLD BY PROGRAM";
String str2 = "Change the world by program";
System.out.println(str1.toLowerCase());
//change the world by program
System.out.println(str2.toUpperCase());
//CHANGE THE WORLD BY PROGRAM

 

public String replace(char oldChar, char newChar)　　//用字符newChar全部替换字符串中的字符oldChar

public String replace(CharSequence target, CharSequence replacement) 　　//用字符串replacement替换原字符串中的字符串target

String str = "Change the world by program";
System.out.println(str.replace('a', 'k'));
//Chknge the world by progrkm
System.out.println(str.replace("the", "a"));
//Change a world by program

public String replaceAll(String regex, String replacement)　　//用字符串replacement替换给定正则表达式(regex)匹配的所有子字符串
public String replaceFirst(String regex, String replacement)　　//用字符串replacement替换给定正则表达式(regex)匹配的第一个子字符串

String str = "Change the world by program";
System.out.println(str.replaceAll(".h.", "EAE"));
//EAEnge EAE world by program
System.out.println(str.replaceFirst(".h.", "EAE"));
//EAEnge the world by program

public String trim()　　//删除字符串前后的空白符

String str = "  Change the world by program  ";
System.out.println(str.trim());
//Change the world by program

 

public String[] split(String regex, int limit)　　//以匹配给定正则表达式分割字符串，以数组形式返回

String str = "Change the world by program";
String[] result = str.split("a|o");　　　　
System.out.println(Arrays.toString(result));
//[Ch, nge the w, rld by pr, gr, m]

String[] result2 = str.split("a|o",3);        //限制份数       
System.out.println(Arrays.toString(result2));
//[Ch, nge the w, rld by program]    

 

public char[] toCharArray()　　//将字符串转化为一个新的字符数组

String str = "Change the world by program";
char[] result = str.toCharArray();
System.out.println(Arrays.toString(result));
//[C, h, a, n, g, e,  , t, h, e,  , w, o, r, l, d,  , b, y,  , p, r, o, g, r, a, m]

 

一些判断方法

public boolean equals(Object anObject)　　//进行比较，如果给定对象与字符串相等，则返回 true；否则返回 false

public boolean equalsIgnoreCase(String anotherString)　　//将此字符串与另一字符串比较，忽略大小写

String str = "Change the world by program";
String str2 = "Change the world by program";
String str3 = "CHANGE THE WORLD BY PROGRAM";
        
System.out.println(str.equals(str2));    //true
System.out.println(str.equals(str3));    //false
System.out.println(str.equalsIgnoreCase(str3));　　//true

 

public boolean contains(CharSequence s)　　//当且仅当此字符串包含指定的字符串时才返回true

String str = "Change the world by program";
System.out.println(str.contains("Change"));    //true
System.out.println(str.contains("change"));    //false

 

public boolean endsWith(String suffix)　　//判断此字符串是否以指定的后缀结尾

public boolean startsWith(String prefix)　　//判断此字符串是否以指定的前缀开始

String str = "Change the world by program";
System.out.println(str.endsWith("program"));    //true
System.out.println(str.endsWith("by"));    //false

System.out.println(str.startsWith("Change"));　　//true
System.out.println(str.startsWith("the"));　　　　//false
System.out.println(str.startsWith("the",7));　　　//也可指定开始查找的位置，因此是true

 

public int compareTo(String anotherString)　　//将此字符串与目标字符串按字典顺序进行比较，根据大小返回不同的值

String str = "ChangE the world by program";
String str2 = "ChangG the world by program";
String str3 = "ChangA the world by program";
String str4 = "ChangE the world by program";
        
System.out.println(str.compareTo(str2));    //比给定字符串小，返回负数-2
System.out.println(str.compareTo(str3));    //比给定字符串大，返回正数4
System.out.println(str.compareTo(str4));    //与给定字符串相等，返回0

 

public boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)

public boolean regionMatches(int toffset, String other, int ooffset, int len)

//比较两个字符串在一段区域内是否相等   ignoreCase若为true，则忽略大小写

String str = "Change the world by program";
String other = "How beautiful the world is";
System.out.println(str.regionMatches(7, other, 14, 9));    //true
//第一个参数toffset为此字符串中子区域的起始偏移量
//第二个参数other为进行比较的字符串
//第三个参数为other中子区域的起始偏移量
//第四个参数len为比较区域长度