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一、if 语句
1. 基本if-else语句
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); } } }
2. 常用比较运算符
(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); } }
3. if-else连写：
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"); } } }
二、条件表达式
(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"); } }
三、switch 语句
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); }
}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); } }