Normally, we are using the following ways to generate a random number in Java.
1. ThreadLocalRandom (JDK 1.7)
//Generate number between 0-9
int index = ThreadLocalRandom.current().nextInt(10);
2. Random()
//Generate number between 0-9
Random random = new Random();
int index = random.nextInt(10);
3. Math.random()
//Generate number between 0-9
int index = (int)(Math.random()*10);
1. For single thread, there is not much performance difference, just pick whatever you want.
2. For multiple threads, it’s recommended to use ThreadLocalRandom. Random is thread safe, but if multiple threads use the same instance of Random, it leads high contention (multiple threads to keep accessing the same “random” generator method) and it kills performance. ThreadLocalRandom solve this by generating a Random instance per thread.
Read this ThreadLocalRandom JavaDoc.
In this tutorial, we will show you how to use above methods to get a random item from a List.
1. ThreadLocalRandom
ThreadLocalRandom example to get a random item from an ArrayList.
package com.mkyong;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;
public class ThreadLocalRandomExample {
public static void main(String[] args) {
List<Integer> list = new ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(40);
list.add(50);
ThreadLocalRandomExample obj = new ThreadLocalRandomExample();
for(int i = 0; i < 10; i++){
System.out.println(obj.getRandomList(list));
}
}
public int getRandomList(List<Integer> list) {
//0-4
int index = ThreadLocalRandom.current().nextInt(list.size());
System.out.println("\nIndex :" + index );
return list.get(index);
}
}
Output. The result will be different each time program is executed.
Index :2
30
Index :4
50
Index :4
50
Index :3
40
Index :4
50
Index :0
10
Index :2
30
Index :1
20
Index :2
30
Index :4
50
2. Ramdom()
Random example to get a random item from an ArrayList.
package com.mkyong;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class RandomExample {
private Random random = new Random();
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("Apple");
list.add("Boy");
list.add("Cat");
list.add("Dog");
list.add("Elephant");
RandomExample obj = new RandomExample();
for(int i = 0; i < 10; i++){
System.out.println(obj.getRandomList(list));
}
}
public String getRandomList(List<String> list) {
//0-4
int index = random.nextInt(list.size());
System.out.println("\nIndex :" + index );
return list.get(index);
}
}
Output
Index :3
Dog
Index :1
Boy
Index :2
Cat
Index :2
Cat
Index :4
Elephant
Index :1
Boy
Index :2
Cat
Index :0
Apple
Index :0
Apple
Index :3
Dog
3. Math.random()
Math.random() example to get a random item from an ArrayList.
package com.mkyong;
import java.util.ArrayList;
import java.util.List;
public class MathRandomExample {
public static void main(String[] args) {
List<Integer> list = new ArrayList<Integer>();
list.add(10);
list.add(20);
list.add(30);
list.add(40);
list.add(50);
MathRandomExample obj = new MathRandomExample();
for(int i = 0; i < 10; i++){
System.out.println(obj.getRandomList(list));
}
}
public int getRandomList(List<Integer> list) {
//Math.random() = greater than or equal to 0.0 and less than 1
//0-4
int index = (int)(Math.random()*list.size());
System.out.println("\nIndex :" + index );
return list.get(index);
}
}
I understand whatever returned from the list wouldn’t be unique values when run in a loop. Right?
And what if I don’t want the index, just the number.
I did that to generate numbers from 1 to 60 and get only six returned
but I couldn’t get ride of the index.
int num = 1;
List list = new ArrayList();
while (num <= 60) {
list.add(num);
num++;
}
ThreadLocalRandomExample obj = new ThreadLocalRandomExample();
for(int i = 1; i < 6; i++) {…..
Is very good your tutorial, and thanks from Domincan Republic
Hi, how would I do it if I want to return 3 random items? thanks.
I really like your Tutorials. Thanks for all the effort. I slightly parameterized this code for reuse.
public T retrieveRandomItemFromList(List t){
int index = ThreadLocalRandom.current().nextInt(t.size());
return t.get(index);
}
I using RandomUtils of apache lang
I think it would have been way more interesting to make performance tests instead of showing obvious outputs.
I agree, which one is the best?
i think current example is more than enough considering the title