字符串的常用方法

Posted 2020-11-20

判断方法

public class Demo4_StringMethod {

    /**
        * boolean equals(Object obj):比较字符串的内容是否相同,区分大小写
        * boolean equalsIgnoreCase(String str):比较字符串的内容是否相同,忽略大小写
        * boolean contains(String str):判断大字符串中是否包含小字符串
        * boolean startsWith(String str):判断字符串是否以某个指定的字符串开头
        * boolean endsWith(String str):判断字符串是否以某个指定的字符串结尾
        * boolean isEmpty():判断字符串是否为空。
        * 
        * ""和null的区别
        * ""是字符串常量,同时也是一个String类的对象,既然是对象当然可以调用String类中的方法
        * null是空常量,不能调用任何的方法,否则会出现空指针异常,null常量可以给任意的引用数据类型赋值
     */
    public static void main(String[] args) {
        //demo1();
        //demo2();
        String s1 = "heima";
        String s2 = "";
        String s3 = null;

        System.out.println(s1.isEmpty());
        System.out.println(s2.isEmpty());
        System.out.println(s3.isEmpty());   //java.lang.NullPointerException
    }

    private static void demo2() {
        String s1 = "我爱heima,哈哈";
        String s2 = "heima";
        String s3 = "baima";
        String s4 = "我爱";
        String s5 = "哈哈";

        System.out.println(s1.contains(s2));        //判断是否包含传入的字符串
        System.out.println(s1.contains(s3));

        System.out.println("------------------");
        System.out.println(s1.startsWith(s4));      //判断是否以传入的字符串开头
        System.out.println(s1.startsWith(s5));

        System.out.println("------------------");
        System.out.println(s1.endsWith(s4));        //判断是否以传入的字符串结尾
        System.out.println(s1.endsWith(s5));
    }

    private static void demo1() {
        String s1 = "heima";
        String s2 = "heima";
        String s3 = "HeiMa";

        System.out.println(s1.equals(s2));      //true
        System.out.println(s2.equals(s3));      //false

        System.out.println("---------------");

        System.out.println(s1.equalsIgnoreCase(s2));    
        System.out.println(s1.equalsIgnoreCase(s3));    //不区分大小写
    }

}

判断

public class Demo3_String {

    /**
     * * 1.判断定义为String类型的s1和s2是否相等
            * String s1 = "abc";
            * String s2 = "abc";
            * System.out.println(s1 == s2);                     
            * System.out.println(s1.equals(s2));        
        * 2.下面这句话在内存中创建了几个对象?
            * String s1 = new String("abc");            
        * 3.判断定义为String类型的s1和s2是否相等
            * String s1 = new String("abc");            
            * String s2 = "abc";
            * System.out.println(s1 == s2); ?           
            * System.out.println(s1.equals(s2)); ?  
        * 4.判断定义为String类型的s1和s2是否相等
            * String s1 = "a" + "b" + "c";
            * String s2 = "abc";
            * System.out.println(s1 == s2); ?           
            * System.out.println(s1.equals(s2)); ?  
        * 5.判断定义为String类型的s1和s2是否相等
            * String s1 = "ab";
            * String s2 = "abc";
            * String s3 = s1 + "c";
            * System.out.println(s3 == s2);
            * System.out.println(s3.equals(s2)); ?  
             */
    public static void main(String[] args) {
        //demo1();
        //demo2();
        //demo3();
        //demo4();
        String s1 = "ab";
        String s2 = "abc";
        String s3 = s1 + "c";
        System.out.println(s3 == s2);
        System.out.println(s3.equals(s2));      //true
    }

    private static void demo4() {
        //byte b = 3 + 4;                       //在编译时就变成7,把7赋值给b,常量优化机制
        String s1 = "a" + "b" + "c";
        String s2 = "abc";
        System.out.println(s1 == s2);           //true,java中有常量优化机制 
        System.out.println(s1.equals(s2));      //true
    }

    private static void demo3() {
        String s1 = new String("abc");          //记录的是堆内存对象的地址值     
        String s2 = "abc";                      //记录的是常量池中的地址值
        System.out.println(s1 == s2);           //false
        System.out.println(s1.equals(s2));      //true
    }

    private static void demo2() {
        //创建几个对象
        //创建两个对象,一个在常量池中,一个在堆内存中
        String s1 = new String("abc");      
        System.out.println(s1);
    }

    private static void demo1() {               //常量池中没有这个字符串对象,就创建一个,如果有直接用即可
        String s1 = "abc";
        String s2 = "abc";
        System.out.println(s1 == s2);           //true  
        System.out.println(s1.equals(s2));      //true
    }

}