Java Collections frequency()

frequency() 可以统计出某个对象在Collection中出现的次数。

1 语法

public static int frequency(Collections<?> c, Object obj)  

2 参数

c：需要查询的集合。

obj：要查询出现频率的对象。

3 返回值

返回对象出现频率数。

4 Collections frequency()示例1

package com.yiidian;

/**
 * 一点教程网： http://www.yiidian.com
 */
/**
 * Java Collections.frequency的例子
 */
import java.util.*;

public class Demo {

    public static void main(String[] args) {
        //Create a list object
        List<String> arrlist = new ArrayList<String>();
        //Add elements in the list
        arrlist.add("Java");
        arrlist.add("COBOL");
        arrlist.add("Java");
        arrlist.add("C++");
        arrlist.add("Java");
        System.out.println("List of elements: "+arrlist);
        //Count the frequency of the given word
        System.out.println("Frequency of the Word: "+Collections.frequency(arrlist,"Java"));
    }
}

输出结果为：

List of elements: [Java, COBOL, Java, C++, Java]
Frequency of the Word: 3

5 Collections frequency()示例2

package com.yiidian;

/**
 * 一点教程网： http://www.yiidian.com
 */
/**
 * Java Collections.frequency的例子
 */
import java.util.*;

public class Demo {

    public static void main(String[] args) {
        //Create a list object
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(1);
        list.add(3);
        list.add(2);
        list.add(3);
        list.add(4);
        System.out.println("List of elements: "+list);
        System.out.println("\nCount all with frequency:");
        Set<Integer> uniqueSet = new HashSet<Integer>(list);
        for (Integer i : uniqueSet) {
            System.out.println(i + ": " + Collections.frequency(list, i));
        }
    }
}

输出结果为：

List of elements: [1, 2, 1, 3, 2, 3, 4]

Count all with frequency:
1: 2
2: 2
3: 2
4: 1

6 Collections frequency()示例3

package com.yiidian;

/**
 * 一点教程网： http://www.yiidian.com
 */
/**
 * Java Collections.frequency的例子
 */
import java.util.*;

public class Demo {

    public static void main(String[] args) {
        // Let us create an array of integers
        Integer arr[] = {20, 10, 20, 30, 20, 40, 20};
        int freq = Collections.frequency(Arrays.asList(arr), 20);
        System.out.println("Frequency of 20 is: "+freq);
    }
}

输出结果为：

Frequency of 20 is: 4