目录

Math类

三角函数

指数函数 

 取整方法

 其他方法

String类

常见方法

字符串比较方法 

子串和数字与字符串的转换 

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Math类

Math类在java.lang中，不用显式引入。

三角函数

    private static void triangleFunc() {
        double degree = Math.toDegrees(Math.PI / 3);
        System.out.println("Math.PI / 3 = " + degree);
        double radians = Math.toRadians(180);
        System.out.println("degree 180 = " + radians);
        double sin = Math.sin(radians/2);
        System.out.println("sin(pi) = " + sin);
        double cos = Math.cos(radians);
        System.out.println("cos(pi) = " + cos);
        double tan = Math.tan(radians/4);
        System.out.println("tan(pi) = " + tan);

    }

输出结果：

指数函数 

   private static void triangleFunc() {
        double degree = Math.toDegrees(Math.PI / 3);
        System.out.println("Math.PI / 3 = " + degree);
        double radians = Math.toRadians(180);
        System.out.println("degree 180 = " + radians);
        double sin = Math.sin(radians/2);
        System.out.println("sin(pi) = " + sin);
        double cos = Math.cos(radians);
        System.out.println("cos(pi) = " + cos);
        double tan = Math.tan(radians/4);
        System.out.println("tan(pi) = " + tan);

    }

 输出结果：

 取整方法

    private static void intFunc() {
        System.out.println("向上取整 0.2 = " + Math.ceil(0.2));
        System.out.println("向下取整0.8 = " + Math.floor(0.8));
        System.out.println("四舍五入 0.6 = " + Math.rint(0.6));
        System.out.println("四舍五入 0.5 = " + Math.round(0.5));
    }

 输出结果：

 其他方法

    private static void otherFunc() {
        System.out.println("较小的数 = " + Math.min(3,4));
        System.out.println("较大的数 = " + Math.max(3,4));
        System.out.println("相反的数 = " + Math.abs(3));
        System.out.println("随机数 = " + Math.random());
    }

 输出结果：

String类

String是一个引用类型，不是如int, char和foat的基础类型，是一个预定义的类。

常见方法

    private static void normalFunc() {
        String message = " Welcome to Java! ";
        System.out.println("The length of message = " + message.length());
        System.out.println("The 5th char = " + message.charAt(5));
        System.out.println(message.concat("Come on."));
        System.out.println("Upper all char:" + message.toUpperCase());
        System.out.println("Lower all char: " + message.toLowerCase());
        System.out.println(message.trim());

    }

输出结果：

字符串比较方法 

    private static void compareFunc() {
        String s = "Hello";
        System.out.println(s.equals("Hello"));
        System.out.println(s.equalsIgnoreCase("hello"));
        System.out.println(s.compareTo("Hell"));
        System.out.println(s.compareToIgnoreCase("he"));
        System.out.println(s.startsWith("He"));
        System.out.println(s.endsWith("lo"));
        System.out.println(s.contains("ell"));
    }

 输出结果：

子串和数字与字符串的转换 

    private static void otherFunc() {

        String what = "summer";
        System.out.println(what.substring(3));
        System.out.println(what.substring(3, 5));

        String float_num = "3.14";
        String int_num = "314";
        System.out.println(Integer.parseInt(int_num));
        System.out.println(Double.parseDouble(float_num));
    }

 输出结果：