Some common methods in the String class in JAVA

Table of contents

String comparison method:

boolean equals(Object anObject)：

 int compareTo(String s)：

int compareToIgnoreCase(String str)

String search method:

char charAt(int index)：

int indexOf(int ch)：

 int indexOf(int ch, int fromIndex)：

int indexOf(String str)：

int indexOf(String str, int fromIndex)：

int lastIndexOf(int ch)：

int lastIndexOf(int ch, int fromIndex)：

int lastIndexOf(String str)：

int lastIndexOf(String str, int fromIndex)：

String conversion

String.valueOf()：

String toUpperCase();String toLowerCase():

char[] toCharArray()；

String(char value[])：

String replacement method:

String replaceAll(String regex, String replacement)：

String replaceFirst(String regex, String replacement)

String[] split(String regex)：

String substring(int beginIndex, int endIndex)：

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String comparison method:

boolean equals(Object anObject)：

Compares two strings for equality, returns true if equal, otherwise returns false

    public static void main(String[] args) {

        String a = "asdf";
        System.out.println(a.equals("aaa"));
        System.out.println(a.equals("asdf"));
    }

 int compareTo(String s)：

To compare whether two strings are equal, first compare them in dictionary order. If unequal characters appear , directly return the difference in size between the two characters ; if the first k characters are equal (k is the minimum length of the two characters), Returns the difference between the lengths of two strings.

    public static void main(String[] args) {

        String a = "asdf";
        System.out.println(a.compareTo("aaa"));
        System.out.println(a.compareTo("asdf"));
        System.out.println(a.compareTo("asd"));

    }

int compareToIgnoreCase(String str)

Ignore the case of characters for comparison, and the return value rules are:

• First, compare the size according to dictionary order. If there are unequal characters , directly return the size difference of the two characters ;
• If the first k characters are equal (k is the minimum length of two characters), return the difference in length between the two strings.

    public static void main(String[] args) {

        String a = "asdf";
        System.out.println(a.compareToIgnoreCase("aaa"));
        System.out.println(a.compareToIgnoreCase("ASDF"));
        System.out.println(a.compareToIgnoreCase("asd"));

    }

String search method:

char charAt(int index)：

Returns the character at index position . If index is negative or out of bounds, an IndexOutOfBoundsException exception is thrown.

    public static void main(String[] args) {

        String a = "asdf";
        System.out.println(a.charAt(0));
        System.out.println(a.charAt(3));
        System.out.println(a.charAt(5));

    }

 

int indexOf(int ch)：

Returns the position where ch first appears , if not, returns -1

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.indexOf('d'));
        System.out.println(a.indexOf('a'));
        System.out.println(a.indexOf('h'));

    }

 int indexOf(int ch, int fromIndex)：

Find ch starting from the fromIndex position  and return the position of the first occurrence , without returning -1

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.indexOf('d', 3));
        System.out.println(a.indexOf('a', 1));
        System.out.println(a.indexOf('h',0));

    }

int indexOf(String str)：

Returns the position where str first appears , without returning -1

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.indexOf("dd"));
        System.out.println(a.indexOf("ss"));

    }

int indexOf(String str, int fromIndex)：

Find the first occurrence of str starting from the fromIndex position , without returning -1

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.indexOf("dd", 3));
        System.out.println(a.indexOf("ss", 0));

    }

 

int lastIndexOf(int ch)：

Search from back to front , return to the position where ch first appears , if not return -1

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.lastIndexOf('d'));
        System.out.println(a.lastIndexOf('s'));
        System.out.println(a.lastIndexOf('v'));

    }

int lastIndexOf(int ch, int fromIndex)：

Start searching from the fromIndex position, and search from back to front for the position where ch first appears . -1 is not returned.

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.lastIndexOf('d', 2));
        System.out.println(a.lastIndexOf('d', 3));
        System.out.println(a.lastIndexOf('d', 4));

        System.out.println(a.lastIndexOf('g', 5));

    }

 

int lastIndexOf(String str)：

Search from back to front and return the position where str first appears . -1 is not returned.

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.lastIndexOf("dd"));
        System.out.println(a.lastIndexOf("as"));
        System.out.println(a.lastIndexOf("bv"));

    }

 

int lastIndexOf(String str, int fromIndex)：

Find the position where str first appears from back to front . If the subscript of this position is not greater than fromIndex, return, otherwise continue to search forward . None returns -1

    public static void main(String[] args) {

        String a = "asdddf";
        System.out.println(a.lastIndexOf("dd", 3));
        System.out.println(a.lastIndexOf("dd", 2));
        System.out.println(a.lastIndexOf("dd", 1));
        System.out.println(a.lastIndexOf("as", 0));
        System.out.println(a.lastIndexOf("bv", 0));

    }

String conversion

String.valueOf()：

Convert all basic type values ​​to string type

    public static void main(String[] args) {

        String s1 = String.valueOf(1234);
        String s2 = String.valueOf(12.34);
        String s3 = String.valueOf(true);
        String s4 = String.valueOf('a');
        System.out.println(s1);
        System.out.println(s2);
        System.out.println(s3);
        System.out.println(s4);
    }

 

String toUpperCase();
​​​​​​​String toLowerCase():

Returns a new string with the original string converted to uppercase .

Returns a new string with the original string converted to lowercase .

    public static void main(String[] args) {
        String s1 = "heLLo";
        String s2 = "HEllO";
        System.out.println(s1.toUpperCase());
        System.out.println(s2.toLowerCase());
    }

char[] toCharArray()；

String(char value[])：

Convert string to array ; original string will not be affected

Convert array to string ; original array will not be affected

    public static void main(String[] args) {
        String s = "hello";
        
        char[] ch = s.toCharArray();
        System.out.println(Arrays.toString(ch));
        
        String s2 = new String(ch);
        System.out.println(s2);
    }

String replacement method:

String replaceAll(String regex, String replacement)：

Replace all specified content

    public static void main(String[] args) {

        String str = "helloworld" ;

        System.out.println(str.replaceAll("l", "O"));
    }

String replaceFirst(String regex, String replacement)

Replace first content

    public static void main(String[] args) {

        String str = "helloworld" ;

        System.out.println(str.replaceFirst("l", "O"));
    }

String[] split(String regex)：

Split all strings

    public static void main(String[] args) {
        String str = "hello world hello" ;
        String[] result = str.split(" ") ; // 按照空格拆分
        for(String s: result) {
            System.out.println(s);
        }
    }

String substring(int beginIndex, int endIndex)：

Intercept strings within the range of [beginIndex,endIndex)

    public static void main(String[] args) {
        String str = "helloworld" ;

        System.out.println(str.substring(0, 5));
    }