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Java Collections - Collections.checkedCollection() Examples

Java Collections Java Java API 

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Class:

java.util.Collections

java.lang.Objectjava.lang.Objectjava.util.Collectionsjava.util.CollectionsLogicBig

Method:

public static <E> Collection<E> checkedCollection(Collection<E> c,
                                                  Class<E> type)

Returns a dynamically typesafe view of the specified collection.


Examples


Not using checkedCollection: 

package com.logicbig.example.collections;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;

public class CheckedCollectionExample {

    public static void main(String... args) {
        Collection<String> c = new ArrayList<>();
        Collections.addAll(c, "apple", "banana");
        System.out.println(c);

        Collection c2 = c;
        c2.add(1);
        System.out.println(c2);
    }
}

Output

[apple, banana]
[apple, banana, 1]




Using checkedCollection: 

package com.logicbig.example.collections;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;

public class CheckedCollectionExample2 {

    public static void main(String... args) {
        Collection<String> c = new ArrayList<>();
        c = Collections.checkedCollection(c, String.class);
        Collections.addAll(c, "apple", "banana");
        System.out.println(c);

        Collection c2 = c;
        c2.add(1);//this throws exception
        System.out.println(c2);
    }
}

Output

Caused by: java.lang.ClassCastException: Attempt to insert class java.lang.Integer element into collection with element type class java.lang.String
	at java.util.Collections$CheckedCollection.typeCheck(Collections.java:3037)
	at java.util.Collections$CheckedCollection.add(Collections.java:3080)
	at com.logicbig.example.collections.CheckedCollectionExample2.main(CheckedCollectionExample2.java:22)
	... 6 more

See Also