java新手笔记24 Math/String对象

1.Math

package com.yfs.javase;

public class MathDemo {

	public static void main(String[] args) {

		int r = 2;

		System.out.println("圆的面积 : " + (Math.PI * r  * r));

		System.out.println("半径的平方 : " + (Math.pow(r, 2)));

		System.out.println("16的平方根 : " + (Math.sqrt(16)));

		System.out.println("随机数 ： " + Math.random());

		System.out.println("Math.round(3.4) ： " + Math.round(3.4));

		System.out.println("Math.round(3.5) ： " + Math.round(3.5));

		System.out.println("Math.ceil(3.1) ： " + Math.ceil(3.1));

		System.out.println("Math.ceil(3.7) ： " + Math.ceil(3.7));

		System.out.println("Math.floor(3.1) ： " + Math.floor(3.1));

		System.out.println("Math.floor(3.7) ： " + Math.floor(3.7));

	}

}

2.String

package com.yfs.javase;

public class StringDemo {

	public static void main(String[] args) {

		String s1 = "  Hello World hello Java I like JAVA you like jAVA   ";

		System.out.println("s1： " + s1);

		System.out.println("s1的长度： " + s1.length());

		//去空格

		s1 = s1.trim();

		System.out.println("s1的长度： " + s1.length());

		System.out.println("s1: " + s1);

		System.out.println("s1第一个字符: " + s1.charAt(0));

		System.out.println("s1最后一个字符: " + s1.charAt(47));

		//查找

		System.out.println("s1大写 : " + s1.toUpperCase());

		System.out.println("s1小写 : " + s1.toLowerCase());

		System.out.println("s1是否包含'java' : " + s1.contains("java"));

		System.out.println("s1是否包含'java' : " + s1.toLowerCase().contains("java"));

		//查找索引

		System.out.println("s1.indexOf('java') : " + s1.toLowerCase().indexOf("java"));

		System.out.println("s1.indexOf('java',19) : " + s1.toLowerCase().indexOf("java",19));

		System.out.println("s1.lastIndexOf('java') : " + s1.toLowerCase().lastIndexOf("java"));

		//比较

		String s2 = new String("aBc");

		String s3 = new String("abC");

		System.out.println("s2.equals(s3) : " + s2.equals(s3));

		System.out.println("s2.equalsIgnoreCase(s3) : " + s2.equalsIgnoreCase(s3));

		System.out.println("s3.compareTo(s2) : " + s3.compareTo(s2));

		System.out.println("s2.compareToIgnoreCase(s3) : " + s2.compareToIgnoreCase(s3));

        //包含字符

		String s4 = "C:\\javase\\Hello.java";

		System.out.println("s4在c盘？" + s4.startsWith("C:"));

		System.out.println("s4是否java文件？" + s4.endsWith(".java"));

		//截取

		s4 = s1.substring(18);

		System.out.println("s4 = " + s4);

		System.out.println("s1.substring(6,11) = " + s1.substring(6,11));

		s4 = "2014-7-1";

		String[] as = s4.split("-");

		for (int i = 0; i < as.length; i++) {

			System.out.println(as[i]);

		}

		//替换

		System.out.println(s1.replace("l", "*"));

		int a = 15;

		String s5 = String.valueOf(a);//类型转换

	}

}

3.Integer

package com.yfs.javase;

public class WrapDemo {

	public static void main(String[] args) {

		int a = 15;

		//a.toString();

		//包装

		Integer o1 = new Integer(a);//整型对象

		String s1 = o1.toString();

		System.out.println("s1 = " + s1);

        //访问属性

		System.out.println("最大值： " + Integer.MAX_VALUE);

		int b = 15;

		Integer o2 = new Integer(b);

		System.out.println("o1 == o2 ? " + (o1 == o2));//比较对象

		System.out.println("a == b ? " + (a == b));//比较数值

		Integer o3 = 15;//自动装箱机制new Integer(15);

		System.out.println("o3 =  " + o3);

		System.out.println("o3 == b ? " + (o3 == b));//自动拆箱

		a = o3;//自动拆箱   65   97

		char c = 'z';

		Character oc = new Character(c);

		System.out.println("oc.compareTo('Z')  : " + oc.compareTo('Z'));

	}

}