1. java.util.Random
这random().nextint(int bound)会生成一个从 0(包括)到 bound(不包括)的随机整数。
(1)代码片段
对于getRandomNumberInRange(5, 10)，这将生成一个介于 5(含)和 10(含)之间的随机整数。
private static int getRandomNumberInRange(int min, int max) {
if (min >= max) {
throw new IllegalArgumentException("max must be greater than min");
}
Random r = new Random();
return r.nextInt((max - min) + 1) + min;
}(2)什么是 (max – min) + 1) + min?
上面的公式将生成一个介于 min(含)和 max(含)之间的随机整数。
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//Random().nextInt(int bound) = Random integer from 0 (inclusive) to bound (exclusive) //1. nextInt(range) = nextInt(max - min) new Random().nextInt(5); // [0...4] [min = 0, max = 4] new Random().nextInt(6); // [0...5] new Random().nextInt(7); // [0...6] new Random().nextInt(8); // [0...7] new Random().nextInt(9); // [0...8] new Random().nextInt(10); // [0...9] new Random().nextInt(11); // [0...10] //2. To include the last value (max value) = (range + 1) new Random().nextInt(5 + 1) // [0...5] [min = 0, max = 5] new Random().nextInt(6 + 1) // [0...6] new Random().nextInt(7 + 1) // [0...7] new Random().nextInt(8 + 1) // [0...8] new Random().nextInt(9 + 1) // [0...9] new Random().nextInt(10 + 1) // [0...10] new Random().nextInt(11 + 1) // [0...11] //3. To define a start value (min value) in a range, // For example, the range should start from 10 = (range + 1) + min new Random().nextInt(5 + 1) + 10 // [0...5] + 10 = [10...15] new Random().nextInt(6 + 1) + 10 // [0...6] + 10 = [10...16] new Random().nextInt(7 + 1) + 10 // [0...7] + 10 = [10...17] new Random().nextInt(8 + 1) + 10 // [0...8] + 10 = [10...18] new Random().nextInt(9 + 1) + 10 // [0...9] + 10 = [10...19] new Random().nextInt(10 + 1) + 10 // [0...10] + 10 = [10...20] new Random().nextInt(11 + 1) + 10 // [0...11] + 10 = [10...21] // Range = (max - min) // So, the final formula is ((max - min) + 1) + min //4. Test [10...30] // min = 10 , max = 30, range = (max - min) new Random().nextInt((max - min) + 1) + min new Random().nextInt((30 - 10) + 1) + 10 new Random().nextInt((20) + 1) + 10 new Random().nextInt(21) + 10 //[0...20] + 10 = [10...30] //5. Test [15...99] // min = 15 , max = 99, range = (max - min) new Random().nextInt((max - min) + 1) + min new Random().nextInt((99 - 15) + 1) + 15 new Random().nextInt((84) + 1) + 15 new Random().nextInt(85) + 15 //[0...84] + 15 = [15...99] //Done, understand?
(3)生成 10 个随机整数的完整示例，范围在 5(含)和 10(含)之间。
package com.mkyong.example.test;
import java.util.Random;
public class TestRandom {
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
System.out.println(getRandomNumberInRange(5, 10));
}
}
private static int getRandomNumberInRange(int min, int max) {
if (min >= max) {
throw new IllegalArgumentException("max must be greater than min");
}
Random r = new Random();
return r.nextInt((max - min) + 1) + min;
}
}输出
761089571085
2. 数学.随机
这Math.random()给出了一个从 0.0(包括)到 1.0(不包括)的随机双精度值。
(1)代码片段。参考1.2，或多或少是相同的公式。
(int)(Math.random() * ((max - min) + 1)) + min
(2)生成16(含)到20(含)范围内的10个随机整数的完整示例。
package com.mkyong.example.test;
public class TestRandom {
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
System.out.println(getRandomNumberInRange(16, 20));
}
}
private static int getRandomNumberInRange(int min, int max) {
if (min >= max) {
throw new IllegalArgumentException("max must be greater than min");
}
return (int)(Math.random() * ((max - min) + 1)) + min;
}
}输出
17
16
20
19
20
20
20
17
20
16
3. Java 8 Random.ints
在 Java 8 中，添加了新方法java.util.Random
public IntStream ints(int randomNumberOrigin, int randomNumberBound) public IntStream ints(long streamSize, int randomNumberOrigin, int randomNumberBound)
这Random.ints(int origin, int bound)或Random.ints(int min, int max)生成一个从原点(包括)到绑定(不包括)的随机整数。
(1)代码片段。
private static int getRandomNumberInRange(int min, int max) {
Random r = new Random();
return r.ints(min, (max + 1)).findFirst().getAsInt();
}(2)生成 33(含)到 38(含)范围内的 10 个随机整数的完整示例。
package com.mkyong.form.test;
import java.util.Random;
public class TestRandom {
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
System.out.println(getRandomNumberInRange(33, 38));
}
}
private static int getRandomNumberInRange(int min, int max) {
Random r = new Random();
return r.ints(min, (max + 1)).limit(1).findFirst().getAsInt();
}
}输出
34
35
37
33
38
37
34
35
36
37
(3)额外，供自己参考。
生成范围在 33(含)和 38(不含)之间的随机整数，流大小为 10。并打印出带有forEach.
//Java 8 only new Random().ints(10, 33, 38).forEach(System.out::println);
输出
34
37
37
34
34
35
36
33
37
34
