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当条件成立时,执行某些语句;否则执行另一些语句。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt();
-
- if (a > 5) {
- System.out.printf("%d is big!\n", a);
- System.out.printf("%d + 1 = %d\n", a, a + 1);
- } else {
- System.out.printf("%d is small!\n", a);
- System.out.printf("%d - 1 = %d\n", a, a - 1);
- }
- }
- }
else 语句可以省略:
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt();
-
- if (a > 5) {
- System.out.printf("%d is big!\n", a);
- System.out.printf("%d + 1 = %d\n", a, a + 1);
- }
- }
- }
当只有一条语句时,大括号可以省略:
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt();
-
- if (a > 5)
- System.out.printf("%d is big!\n", a);
- else
- System.out.printf("%d is small!\n", a);
- }
- }
练习:输入一个整数,输出这个数的绝对值。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int x = sc.nextInt();
-
- if (x > 0)
- System.out.println(x);
- else
- System.out.println(-x);
- }
- }
练习:输入两个整数,输出两个数中较大的那个。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt(), b = sc.nextInt();
-
- if (a > b)
- System.out.println(a);
- else
- System.out.println(b);
- }
- }
if-else语句内部也可以是if-else语句。
练习:输入三个整数,输出三个数中最大的那个。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();
-
- if (a > b) {
- if (a > c)
- System.out.println(a);
- else
- System.out.println(c);
- } else {
- if (b > c)
- System.out.println(b);
- else
- System.out.println(c);
- }
- }
- }
(1) 大于 >
(2) 小于 <
(3) 大于等于 >=
(4) 小于等于 <=
(5) 等于 ==
(6) 不等于 !=
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt(), b = sc.nextInt();
-
- if (a > b) System.out.printf("%d > %d\n", a, b);
- if (a >= b) System.out.printf("%d >= %d\n", a, b);
- if (a < b) System.out.printf("%d < %d\n", a, b);
- if (a <= b) System.out.printf("%d <= %d\n", a, b);
- if (a == b) System.out.printf("%d == %d\n", a, b);
- if (a != b) System.out.printf("%d != %d\n", a, b);
- }
- }
输入一个0到100之间的分数,
如果大于等于85,输出A;
如果大于等于70并且小于85,输出B;
如果大于等于60并且小于70,输出C;
如果小于60,输出 D;
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int s = sc.nextInt();
-
- if (s >= 85) {
- System.out.println("A");
- } else if (s >= 70) {
- System.out.println("B");
- } else if (s >= 60) {
- System.out.println("C");
- } else {
- System.out.println("D");
- }
- }
- }
练习:
1.判断闰年。闰年有两种情况:
(1) 能被100整除时,必须能被400整除;
(2) 不能被100整除时,被4整除即可。
输入一个年份,如果是闰年输出yes,否则输出no。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int year = sc.nextInt();
-
- if (year % 100 == 0) {
- if (year % 400 == 0)
- System.out.println("yes");
- else
- System.out.println("no");
- } else {
- if (year % 4 == 0)
- System.out.println("yes");
- else
- System.out.println("no");
- }
- }
- }
(1) 与 &&
(2) 或 ||
(3) 非 !
例题:输入三个数,输出三个数中的最大值。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();
-
- if (a >= b && a >= c)
- System.out.println(a);
- else if (b >= a && b >= c)
- System.out.println(b);
- else
- System.out.println(c);
- }
- }
练习:用一条if语句,判断闰年。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int year = sc.nextInt();
-
- if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
- System.out.println("yes");
- else
- System.out.println("no");
- }
- }
注意: swtich语句中如果不加break语句,则从上到下匹配到第一个case后,会顺次执行后面每个case中的语句。
- import java.util.Scanner;
-
- public class Main {
- public static void main(String[] args) {
- Scanner sc = new Scanner(System.in);
- int day = sc.nextInt();
- String name;
-
- switch(day) {
- case 1:
- name = "Monday";
- break;
- case 2:
- name = "Tuesday";
- break;
- case 3:
- name = "Wednesday";
- break;
- case 4:
- name = "Thursday";
- break;
- case 5:
- name = "Friday";
- break;
- case 6:
- name = "Saturday";
- break;
- case 7:
- name = "Sunday";
- break;
- default:
- name = "not valid";
- }
-
- System.out.println(name);
- }
- }