List.contains() 和set.contains()

set 检索效率低，插入和删除元素效率高。
list 检索效率高，插入和删除元素效率低。

list调用contains方法时 ，会遍历所有元素，并调用equals()方法，equals（）方法返回true, 则contains 返回ture ,时间复杂度为O(n)

set调用contains方法时，会比较所有元素的hashcode方法，当hashcode方法返回true时再比较equals ,也返回true时，contains返回true,。时间复杂度为O(1) 。

public class Dog {

private String id;
private String name;

public Dog(String id, String name) {
    this.id = id;
    this.name = name;
}

public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

@Override
public boolean equals(Object obj) {
    Dog dog = (Dog) obj;
    if(id.equals(dog.getId())&&
     name.equals(dog.getName())){
        return true;
    }else{
        return false;
    }
}

@Override
public int hashCode() {
    return (id+"_"+name).hashCode();
}

}

@Test
public void listContains(){
List<Dog> list = new ArrayList<>();
list.add(new Dog("1","we"));
list.add(new Dog("2","erer"));

    System.out.println(list.contains(new Dog("1","we")));
}

@Test
public void setContains(){
    Set<Dog> set = new HashSet<>();
    set.add(new Dog("1","we"));
    set.add(new Dog("2","erer"));

    System.out.println(set.contains(new Dog("1","we")));
}

源码

list

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

set

final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}