Java从零开始学七(选择结构)

一、 程序的结构:

一般来说程序的结构包含有下面三种：

1、顺序结构

2、选择结构

3、循环结构

二、顺序结构

程序至上而下逐行执行，一条语句执行完之后继续执行下一条语句，一直到程序的末尾

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三、选择结构

选择结构是根据条件的成立与否，再决定要执行哪些语句的一种结构

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3.1 、IF语句--单分支结构

if (判断条件){

语句1 ；

…

语句2 ；

}

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示例一、 比较2个整型变量的大小

package pb.test;

public class test4 {

    public static void main(String[] args) {
        int x = 3;
        int y = 10;          //定义2个整型变量，x,y
        System.out.println("=======开始比较=======");
        if(x>y){
            System.out.println("x比y大!");
        }
        if(x<y){
            System.out.println("x小于y!");
        }
        if(x==y){
            System.out.println("x等于y!");
        }
        System.out.println("=======比较完成=======");

    }

}

3.2、if...else语句--双分支结构

if (判断条件){

语句主体1 ；

}else{

语句主体2；

}

aaarticlea/png;base64,iVBORw0KGgoAAAANSUhEUgAAANUAAACdCAIAAACsKarVAAALbUlEQVR4nO2dPWgTbxzHn0k7qAQFzaAQt06SoUMHlSoKWSpBRERROklAhwiCggHdiiDcJIU6OGYKgYIEp0NEsliSTjVTCB1C6NBCKaGUcv/h+B9Pn7t7nt8l99xbvp8hXHJ3z8vlc997TY5ZKWFpaYkx1u/3424ICBMWdwOowL9MAv9AnMA/ECfwD8QJ/ANxAv9AnKTAv06nY5pmsVhkjNXrddM0x+Nx3I0C4ZAC/wzDYBzFYjHuFoHQSIF/4/E4n887/jWbzbhbBEIjBf5ZXAQi/DJGOvxzIhDhlzHS4Z9lWYZhIPyyRzr8a7Var1+/vnv37vfv34fDYdzNAaGRXP/6/f7a2lq5XJ6bmyuVSoZh1Ov1lZWVfD5fLBbfv3/fbrfjbiOYlsT512q1qtXq/Px8oVCoVCrNZtN9tq/T6ayuri4uLuZyuSdPniAU00si/HNH3fb2NmXGvb09hGKqidM/StTRQSimkaj9mzjq6GQpFO2rjpnBfQYjIv/CjTo6aQ9FxhKxgxQW7u5o7F4EUUcnpaEI/wITV9TRSVEowj8SiYo6OskPRfjny3g8Tn7U0UlmKMI/ke3tbcMwSqVSoKjzXI7ED+1DJ8q88mmI32WiQjF7/vVPQ+reeDxuNpuVSqVQKMzPz1er1VarRazPb8BvmB/wNE8YK5/Gr2oisYdiWP5NsLpKxk7cKsaYeRpZQXzUlcvltbW1CX5+wUsgqODuEu+TpJOUYJOrH5TIQrHT6fBvlW32O9OmLIT+oUWQj7hs1f5NHHXKit1vlfknKYo4meX1DU3cCxutocgYK5fLjoWBWkvcVhCXg59znt/UtP6FEnV+Vbq77emfHKHP8m4LFbnnDQXPUDRN8+MUOP21LQzqn9stySLynNHTTua1MvOjpvKvWq0yxnK5nGEY9N4GwrM/yrXKc17rtHl+C8tzril6oMD5hR7/U5VQkNdLWYaeBXqW7Pchc/lHmdGzKO/8030mhegf3z3PpSl0Xjlqsi+Vjntb3Gg0Qsm/YrHYbDbp/vH9tXy8dE8vL0343P3qWYW8tYr9v3DPJPt9957LSBglWVIUpSifTIa92X3x4sWVK1du3LgR7rGIY57zVjk9P+y5NCj+URam8K35KS5vbYDj3xBDUbI+CRNIBpTFykdN6Z8ddQsLCxcuXLh///67d+8ajYZpmn/+/Nne3h6NRsfHx9OUbyP8wIronyANxS15/rk/5yuSKy5vbQD/bI6Pj//+/fvx48c7d+6cPXv23r17QUNR0ivJNPL84weIi3IC/+yoe/78+eXLl+fn5589e/b161fTn83NzcFgcHBwELQiPyhtlqzMQUue7Jsi1mgF8u/g4GAwGGxubgozfP78+fHjx9evX7927drLly+VoUiMKGHFkq/N7uFwt79+UUcnrFAMtM5INqDuyfzm8ixkyk0HXw7Jv4ODg52dna2tLckirtfrb968WVpakoSiXD7K2iYpRL6gg/oXNOqUtNvtXq+3u7t7cnIiqVdOWF98Qgi8/T05Odnd3e31evaZLT+ChmJysKOuWCxOHHUCW1tbOzs7//79+/Lly/TNm3X/eA4PD0MJxdhxou7SpUuaok65ESQC/zxIaSi69+revn07jXZ21B0eHgoVee5R1Wq1Wq1mnVbTGa7X6/bw+vq6UFQUiyYqwvGPJ+GhyO/VFQoFIeo+ffoU1DniXp1bssFg4DfKlm80Go1GI8YYv+HO5XIsQ+RyuZD9c6CH4s2bN3WHoucBrNs2un9+UeeHvbjdw56j3EhK7vf71HXFND9xSCazrxwahuE5l2chE6y6nmiJd0koOu0OPRTtqHv69KnfAWxQ/6Y5gJ3AP2LJgfwjUiqVeP+iRO/uRQR7itOfqxMIGnWeSPxbXl5mjI1Go16vZ49aX19njLXbbff21w38m5AQ9xSTea6Op9vt2p61223BP2fUxsaGM8o5/lCetYF/0zJxKJqmmcCoixj4Fyb0ULx69er58+cfPnyYtKiLmOFw2AmbBw8eMMa+ffsWeslKknJ6SRmK/PMXJiCNURcZKysrjDHTNKOvOin+8XiG4gT+pT3qIgP+ecOHIt0/RF1Q4J+aW7duMcZarRaiLnTgnxrn+W98KCLqQgH+qcHzB/UB/9TI/XNO6nqevw10gWsGgX9q5P7JDYN/cuCfGol/wlX80WhUqVQYY8vLy91u1+L86/V6tVrNudI1Go0s/3vvZgf4p4aef5VKxRbLVpAfa2+j9/f37Sv9tVpNcu/d7AD/1ND929/f39jYcHKOH8uHn/2jceYiov4kCfinJlD+McYGg4HbP8uyBoPBxsaGPY39OpvO8cA/NXT/7GF7e+qZf/zWOdC9d1kF/qmh+9ftdtn/P6oQ/Nvf37eFs3f+hOOP2ZTPgn8UcP5ZH/BPDfzTB/xTA//0Af/UwD99wD818E8f8E9Gp9MR7n9Oy58bpQX4J8MwDP76hPsZsmBK4J+M8XjM/6m88A+1YHrgnwInAhF+OoB/CpwIRPjpAP6pMQwD4aeJdPtnH5lmhhm0PN3+sWzdv5Sx7lCAfwkiY92hAP8SRMa6QwH+JYiMdYcC/EsQGesOBfiXIBhjwn/UHR0dxd0ovcA/ajnyafixE7eKuZ5RkfnbHTLun9+ZNmUh9A8tlXyelfqVA/8iI+r8k9ijdFRemucwC/5sUvgXJTH453aLGGnEBJXrC//cZNw/STJZXES5vQm0UXb7N3Fr4V9kROofr5rlv1kk+ueZf+7XQE214F+0xJB/8q2nJTVMsm31NI+ylXeXA/8iIzr/BGkobgXageNj1e0oUT4L/kVLRMcf8g1uoJIpO3aBgtNdDvyLjEQc//pN5jfXlAmnbCH8i4wEXf9ICPAvSuCfCPyLEvgnAv+iBP6JwL8oCUGdXC7HMsS5c+fgX2Roia5Az0j+xCGZrFAoMO75g8JcnoXIC6QD//QRv39Epnz+7zTAP33APzXwTx/wTw380wf8UwP/9KHFv729vX7YLC4uMsZ+//4deslKjo+PdSyl5JA1/3SA/9/VB/xTA//0Af/UwD99wD818E8f8E8N/NMH/FMD//QB/9TAP33APzXwTx/wTw380wf8k1GtVvn78/L5fOYviEUM/JMxHA7n5uYc/wzDiLtFWQP+KXAiEOGnA/inwIlAhJ8O4J+aarWK8NNEuv3D84/STrr9Y5n7/WXcTYga+JcgMtYdCvAvQWSsOxTgX4LIWHcowL8EkbHuUIB/CYIxht8fRUYM/jHVn0Y6H05s9jSrBMP/D0VInP55eia30++knbI99EYK/v38+bPb7RLnTSkz5B/RHv41aL0UR+XFOua9evXq4sWLHz58IM6bUmbIP0v1F9CCfxJ1PMuRTExvnmOeXfjt27c/Zhr7CtYM+ec3o2do+enlLkewllczkH/5fN4vRDPMDPnn4C7H71Woi3Gq+U3jnp7YvEaj8ejRozNnztjlZz7/bGK5tzwR+ec2hp1OMs+65B5LqlA2z97/cyzM/P5fjETqnxD4FhdgQmnuVBPG+knpWUvQRvLHv41G49evX8R5QVBiPv/n91YZaRL/pmwkm73zfzESQ/4RS/OMMXdpyjIplQrTw7/IwPU3EfgXJfBPBP5FCfwTgX9RAv9E4F+UwD8R+BclIaiD52+BidESXX08fwHQgH9q4J8+4J8a+KcP+KcG/ulDi3/D4bATNgsLC4yxHz9+hF6ykqOjIx1LCVia/NMB/v80k8A/ECfwD8QJ/ANxAv9AnMA/ECfwD8QJ/ANxAv9AnKTAv9XV1aWlJfsur8XFxVKphAtimSEF/nU6Hf7+vGq1GneLQGikwD/Lssrlsi3f3NzccDiMuzkgNNLhnxOBCL+MkQ7/LMsql8sIv+yRGv86nQ7CL3ukxj/LsnDYmz3+A2tUTdrAmUL2AAAAAElFTkSuQmCC" alt="" />

示例二、判断一个数字是奇数还是偶数

package pb.test;

public class test5 {

    public static void main(String[] args) {
        int x=3;           //定义大中型变量x
        if(x%2==0){             //如果余数为0则是偶数
            System.out.println("x是偶数");
        }else{                   //如果余数不为0则是奇数
            System.out.println("x是奇数");
        }

    }

}

3.3、三目运算符

使用三目运算符时，操作数有3个，其格式如下：

变量 = 条件判断？表达式1：表达式2

根据条件的成立与否，来决定结果为“：”前或“：”后的表达式，如果条件判断为true，则执行表达式1，为false则执行表达式2

示例三、求出2个数字中的最大值

public static void main(String[] args) {
        //定义变量来保存最大值
        int max=0;
        //定义2个变量
        int x=3;
        int y=10;
        //使用3目运算判断如果x>y则，max=x,否则max=y;
        max=x>y?x:y;
        System.out.println("max为: "+max);

    }

3.4、if…else if…else语句 --多分支结构

如果需要在if..else里判断多个条件时，就需要if..else if … else语句了，其格式如下：

if (条件判断1){

语句主体1 ；

}else if (条件判断2){

语句主体 2 ；

}

… // 多个else if()语句

else{

语句主体3 ；

}

示例四、

package pb.test;

public class test6 {

    public static void main(String[] args) {
        int x=3;
        if(x==1){
            System.out.println("x的值是1!");
        }else if(x==2){
            System.out.println("x的值是2!");
        }else if(x==3){
            System.out.println("x的值是3!");
        }else{
            System.out.println("x的值不是1，2，3 中的一个!");
        }

    }

}

四、Switch结构

要在许多的选择条件中找到并执行其中一个符合判断条件的语句时，除了可以使用if..else不断地判断之外，也可以使用另一种更方便的方式即多重选择——switch语句，语法格式：

switch (表达式){

case 选择值1 ： 语句主体 1 ；        break ；

case 选择值2 ： 语句主体 2 ；                 break ；

…….

case 选择值n ： 语句主体 n ；                break ； default：  语句主体 ；

}

4.1、执行流程

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" alt="" />

示例五、判断学生成绩，并给于评分，90~100分，A级大于80分，B级，大于70分，输出C级，大于60分，D级，低于60分输出E级

package pb.test;

public class test7 {

    public static void main(String[] args) {
       int score = 75;   //声明学生分数
       switch (score/10){
       case 10:
       case 9:
         System.out.println("A级");
       break;
       case 8:
           System.out.println("B级");
           break;
       case 7:
           System.out.println("C级");
           break;
       case 6:
           System.out.println("D级");
           break;
       default:
           System.out.println("E级");
           break;
       }

    }

}

结果:C级

还是上面的如果不加break;

package pb.test;

public class test7 {

    public static void main(String[] args) {
       int score = 75;   //声明学生分数
       switch (score/10){
       case 10:
       case 9:
         System.out.println("A级");

       case 8:
           System.out.println("B级");

       case 7:
           System.out.println("C级");

       case 6:
           System.out.println("D级");

       default:
           System.out.println("E级");

       }

    }

}

结果：C级,D级,E级