String中常用的API（一）

（1）boolean isEmpty()：字符串是否为空

    @Test
    public void test1(){
        String s1 = new String("");
        boolean b1 = s1.isEmpty();
        System.out.println(b1);//true

        String s2 = new String("123");
        boolean b2 = s2.isEmpty();
        System.out.println(b2);//false
    }

（2）String concat(xx)：拼接，等价于+

    @Test
    public void test2(){
        String s1 = new String("abc");
        String s2 = new String("123");
        String s3 = s1.concat(s2);
        System.out.println(s3);//abc123
    }

（3）int length()：返回字符串的长度

 @Test
    public void test3(){
        String s1 = new String("abc");
        int length = s1.length();
        System.out.println(length);//3
    }

（4）boolean equals(Object obj)：比较字符串是否相等，区分大小写

 @Test
    public void test4(){
        String s1 = new String("abc");
        String s2 = new String("abc");
        String s3 = new String("ABC");
        String s4 = new String("123");

        boolean b1 = s1.equals(s2);
        System.out.println(b1);//true

        boolean b2 = s1.equals(s3);
        System.out.println(b2);//false

        boolean b3 = s1.equals(s4);
        System.out.println(b3);//false
    }

（5）boolean equalsIgnoreCase(Object obj)：比较字符串是否相等，不区分大小写

    @Test
    public void test5(){
        String s1 = new String("abc");
        String s2 = new String("abc");
        String s3 = new String("ABC");
        String s4 = new String("123");

        boolean b1 = s1.equalsIgnoreCase(s2);
        System.out.println(b1);//true

        boolean b2 = s1.equalsIgnoreCase(s3);
        System.out.println(b2);//true

        boolean b3 = s1.equalsIgnoreCase(s4);
        System.out.println(b3);//false
    }

（6）int compareTo(String other)：比较字符串大小，区分大小写，按照Unicode编码值比较大小

    @Test
    public void test6(){
        String s1 = new String("abc");
        String s2 = new String("ABC");
        String s3 = new String("bcd");
        String s4 = new String("狗吉");

        int i1 = s1.compareTo(s2);
        System.out.println(i1);//32

        int i2 = s2.compareTo(s3);
        System.out.println(i2);//-33

        int i3 = s1.compareTo(s3);
        System.out.println(i3);//-1

        int i4 = s1.compareTo(s4);
        System.out.println(i4);//-29302       
    }

（7）int compareToIgnoreCase(String other)：比较字符串大小，不区分大小写

 @Test
    public void test7(){
        String s1 = new String("abc");
        String s2 = new String("ABC");
        String s3 = new String("bcd");
        String s4 = new String("狗吉");

        int i1 = s1.compareToIgnoreCase(s2);
        System.out.println(i1);//0

        int i2 = s2.compareToIgnoreCase(s3);
        System.out.println(i2);//-1

        int i3 = s1.compareToIgnoreCase(s3);
        System.out.println(i3);//-1

        int i4 = s1.compareToIgnoreCase(s4);
        System.out.println(i4);//-29302
    }

（8）String toLowerCase()：将字符串中大写字母转为小写

    @Test
    public void test8(){
        String s1 = new String("AbcDefG");
        String s = s1.toLowerCase();
        System.out.println(s);//abcdefg
    }

（9）String toUpperCase()：将字符串中小写字母转为大写

    @Test
    public void test9(){
        String s1 = new String("AbcDefG");
        String s = s1.toUpperCase();
        System.out.println(s);//ABCDEFG
    }

（10）String trim()：去掉字符串前后空白符

@Test
    public void test10(){
        String s1 = new String("  abc  ");
        String trim1 = s1.trim();
        System.out.println(trim1);//abc

        String s2 = new String("  狗吉  ");
        String trim2 = s2.trim();
        System.out.println(trim2);//狗吉
    }