public class Str {/*** @param String类举例*/public static void main(String[] args) {String a1 = "abc";String a2 = "abc";String a3 = "abcd";System.out.println(a1==a2);//trueSystem.out.println(a1==a3);//false
        System.out.println();String b1 = new String("abc");String b2 = new String("abc");String b3 = new String("abcd");System.out.println(b1==b2);//falseSystem.out.println(b1==b3);//falseSystem.out.println(b1.equals(b2));//trueSystem.out.println(b1.equals(b3));//false
        System.out.println();char [] c = {'a','b','c','d'};String c1 = new String(c);String c2 = new String(c,0,3);//c:字符数组；0：数组中的第一个元素；3：c2的长度System.out.println(c1);//abcdSystem.out.println(c2);//abc
    }
}

public class Test {public static void main(String [] args){String a1 = "Sun JAVA";String a2 = "sun java";System.out.println("a1的长度："+a1.length());System.out.println("a1中第五个字符是："+a1.charAt(5));System.out.println("a1中第一个出现JAVA的位置："+a1.indexOf("JAVA"));System.out.println(a1.equals(a2));System.out.println(a1.equalsIgnoreCase(a2));System.out.println(a1.startsWith("Sun"));//判断是否以指定前缀开始System.out.println(a1.endsWith("JAVA"));//判断是否以指定后缀结尾
        String a3 = "I Love Play Computer Game!";String a4 = a3.replace(' ', '-');System.out.println("a3="+a3);System.out.println("a4="+a4);}
}

　　　　 int b1 = 1234;double b2 = 12.13;boolean b3 = true;//类型转换String b4 = String.valueOf(b1);String b5 = String.valueOf(b2);String b6 = String.valueOf(b3);

public class Text {/*** @param Math类*/public static void main(String[] args) {double a1 = Math.random();double a2 = Math.random();System.out.println(Math.sqrt(a1*a1+a2*a2));System.out.println(Math.pow(a1, 8));System.out.println(Math.round(a1));System.out.println(Math.log(Math.pow(Math.E,15)));}
}

public class file {/*** @param File类*/public static void main(String[] args) {String separator = File.separator;String fileName = "myfile.txt";//文件名
        String directory = "mydir1"+separator+"mydir2";//文件路径//String directory = "mydir1/mydir2";//String directory = "mydir1\\mydir2";
        File f = new File(directory, fileName);if(f.exists()){//判断文件是否存在System.out.println("文件名："+f.getName());System.out.println("文件大小："+f.length());}else{f.getParentFile().mkdirs();//创建文件路径try {f.createNewFile();//创建文件} catch (IOException e) {e.printStackTrace();}}}
}