二、java流程控制语句

一、顺序结构

顺序结构：程序从上到下逐行地执行，中间没有任何判断和跳转

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public class Test {
    public static void main(String[] args) {
                int i = 12;
                int j = i +1;
        System.out.println(j);
    }
}

 

二、分支结构

分支结构：根据条件，选择性的执行某段代码（if、switch）

1、if

（1）第一种格式

public class Testif {
    public static void main(String[] args) {
        int i = 2;
        boolean b = (i == 2);
        if(i == 2) {
            System.out.println("hello");
        }
        //if语句括号中，结果为true，就执行后面大括号中的语句
        if(true) {
            
        }
        //判断i如果是偶数，打印：偶数
        if(i % 2 == 0) {
            System.out.println("偶数");
        }
        //当if语句后面的{}中只有一条语句时，可以省略{}，但是不允许使用
        if(i % 2 == 0) 
            System.out.println("偶数");   
    }
}

（2）第二种格式

public class Testif2 {
    public static void main(String[] args) {
        int i = 3;
        if(i == 2) {
            System.out.println("hello");
        }else {
            System.out.println("java");
        }
        
        int score = 80;
        if(score >= 60) {
            System.out.println("及格");
        }else {
            System.out.println("不及格");
        }
        
        int x = 20;
        if(x % 2 == 0) {
            System.out.println("该数是偶数");
        }else {
            System.out.println("该数是奇数");
        }
    }
}

（3）第三种格式

public class Testif3 {
    public static void main(String[] args) {
        int i = 5;
        if(i == 1) {
            System.out.println("one");
        }else if(i == 2){
            System.out.println("two");
        }else if(i == 3) {
            System.out.println("three");
        }else if(i == 4) {
            System.out.println("four");
        }else {
            System.out.println("other");
        }
        
        
        
        int score = 65;
        if(score > 100 || score <0) {
            System.out.println("错误");
        }else if(score >= 80 && score <100){
            System.out.println("优秀");
        }else if(score >= 60 && score <80) {
            System.out.println("良好");
        }else {
            System.out.println("不及格");
        }
        
        
        score = 49;
        String str;
        if(score > 100 || score <0) {
            str = "错误";
        }else if(score >= 80){
            str = "优秀";
        }else if(score >= 60) {
            str = "良好";
        }else {
            str = "不及格";
        }
        System.out.println(str);
        
        
        
        int age = 11;
        if(age > 150 && age <0) {
            System.out.println("不可能");
        }else if(age >= 60) {
            System.out.println("老年人");
        }else if(age >= 40) {
            System.out.println("中年人");
        }else {
            System.out.println("年轻人");
        }
    }

}

2、switch

public class Testswitch {
    public static void main(String[] args) {
        int i = 2;
        switch(i) {
        case 1:
            System.out.println("one");
            break;
        case 2:
            System.out.println("two");
            break;
        case 3:
            System.out.println("three");
            break;
        default:
            System.out.println("other");
            break;
        }
        
        
        String season = "autumn";
        switch(season) {
        case "spring":
            System.out.println("春天");
            break;
        case "summer":
            System.out.println("夏天");
            break;
        case "autumn":
            System.out.println("秋天");
            break;
        case "winter":
            System.out.println("冬天");
            break;
        default:
            System.out.println("错误");
            break;
        };
    }

}

三、循环结构

循环结构：根据循环条件，重复性的执行某段代码（for、while、do while）

1、for

public class Testfor {
    public static void main(String[] args) {
        for(int i = 1; i <= 5; i++) {
            System.out.println(i);    
        }
        for(int i = 1; i <= 5; i++) {
            System.out.println("******");    
        }
        for(int i = 0; i <= 4; i++) {
            System.out.println("******");    
        }
        for(int i = 0; i < 5; i++) {
            System.out.println("******");    
        }
        for(int i = 4; i >= 0; i--) {
            System.out.println("******");    
        }
        
        int sum = 0;
        for (int a = 1; a <= 100; a++) {    
            sum = sum + a;
        }
        System.out.println(sum);
              
        int sum2 = 0;
        for (int a = 1; a <= 100; a++) {
            if(a % 2 == 0) {
                sum2 = sum2 + a;    
            }    
        }
        System.out.println(sum2);
                    
        sum = 0;
        for (int i = 1; i <= 100; i++) {
            if(i % 3 == 0) {
                sum = sum + i;
            }
        }
        System.out.println(sum);
        
        int count = 0;
        for(int i = 1; i <= 100; i++) {
            if(i % 3 == 0) {
                count = count + 1;
            }
        }
        System.out.println(count);
        
        count = 0;
        for(int i = 1; i <= 100; i++) {
            if(i % 7 == 0) {
                count = count + 1;
            }
        }
        System.out.println(count);
    }

}

2、while

public class Testwhile {
    public static void main(String[] args) {
        int i = 1;
        while(i <= 5) {
            System.out.println(i);
            i++;
        }
        
        
        int sum = 0;
        i = 1;
        while(i <= 100) {
            sum = sum + i;
            i++;
        }
        System.out.println(sum);
        
        sum = 0;
        i = 1;
        while(i <= 100) {
            if(i % 2 == 0) {
                sum = sum + i;
            }
            i++;
        }
        System.out.println(sum);
         
        int count = 0;
        i = 1;
        while(i <= 100) {
            if(i % 3 == 0) {
                count = count + 1;
            }
            i++;
        }
        System.out.println(count);
    }

}

3、do while

public class Testdowhile {
    public static void main(String[] args) {
        int i = 1;
        do {
            System.out.println(i);
            i++;
        }while(i <= 5);
           
        int sum = 0;
        i = 1;
        do {
            sum = sum + i;
            i++;
        }while(i <= 100);
        System.out.println(sum);
         
        sum = 0;
        i = 1;
        do {
            if(i % 2 == 0) {
                sum = sum + i;
            }
            i++;
        }while(i <= 100);
        System.out.println(sum);
        
        int count = 0;
        i = 1;
        do {
            if(i % 7 == 0) {
                count = count + 1;
            }
            i++;
        }while(i <= 100);
        System.out.println(count);
    }

}

四、特殊流程控制语句（break、continue）

1、break

public class Testbreak {
    public static void main(String[] args) {
        for(int i = 1; i <= 5; i++) {
            if(i == 4) {
                break;
            }
            System.out.println(i);
        }
    }
}

2、continue

public class Testcontinue {
    public static void main(String[] args) {
        for(int i = 1; i <= 5; i++) {
            if(i == 3) {
                continue;
            }
            System.out.println(i);
        }
    }
}

五、循环嵌套

public class Testln {
    public static void main(String[] args) {
        for(int i = 1; i <=5 ; i++) {
            for(int j = 1; j <= 6; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
        
        for(int i = 1; i <=5 ; i++) {
            int j = 1;
            while(j <= 6) {
                System.out.print("*");
                j++;
            }
            System.out.println();
        }
        
        for(int i = 1; i <= 4 ; i++) {
            for(int j = 1; j <= i; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
        
        for(int i = 1; i <= 9 ; i++) {
            for(int j = 1; j <= i; j++) {
                System.out.print(j + "*" + i + "=" +i*j + "\t");
            }
            System.out.println();
        }   
    }
}

六、无限循环

public class test {
    public static void main(String[] args ) {
        /*for(;;) {
            System.out.println("-------------------");
        }*/
        /*while(true) {
            System.out.println("-------------------");
        }*/
        /*while(1 == 1) {
            System.out.println("-------------------");
        }*/
        int t = 0;
        while(true) {
            t++;
            if(t == 5) {
                break;
            }
            System.out.println("****************");
        }    
    }
}

 

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