若要計算List中某特定元素有幾個，可使用Collections.frequency(Collection<?> c, Object o)。
第一個餐數c為要計算的集合物件，所以只要是Collection的子類都可以，第二個參數o為要計算的元素。
List<Integer> list = Arrays.asList(1,1,1,2,3,3,4,5);
System.out.println(Collections.frequency(list, 1)); // 3
System.out.println(Collections.frequency(list, 2)); // 1
System.out.println(Collections.frequency(list, 3)); // 2
List<String> list = Arrays.asList("A","B","B","C","C","C");
System.out.println(Collections.frequency(list, "A")); // 1
System.out.println(Collections.frequency(list, "B")); // 2
System.out.println(Collections.frequency(list, "C")); // 3
如果是自訂類別記得要覆寫equals()及hashCode()。
public class Main {
public static void main(String[] args) {
List<People> list = Arrays.asList(
new People("Mario"),
new People("Mario"),
new People("Luigi"),
new People("Toad"),
new People("Toad"),
new People("Toad")
);
System.out.println(Collections.frequency(list, new People("Mario"))); // 2
System.out.println(Collections.frequency(list, new People("Luigi"))); // 1
System.out.println(Collections.frequency(list, new People("Toad"))); // 3
}
}
class People {
private String name;
public People(String name) {
this.name = name;
}
@Override
public boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof People)) {
return false;
}
People p = (People) o;
return p.name.equals(name);
}
@Override
public int hashCode() {
int result = 17;
result = 31 * result + name.hashCode();
return result;
}
}
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