背景

项目中有时候会使用Collections.emptyList返回一个空列表，但是emptyList在执行add，remove等方法时会直接抛出UnsupportedOperationException异常，我们可以看下源码

public class Collections {
    public static final List EMPTY_LIST = new EmptyList<>();

    public static final <T> List<T> emptyList() {
        return (List<T>) EMPTY_LIST;
    }

    private static class EmptyList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable {

        public int size() {return 0;}
        public boolean isEmpty() {return true;}
        public boolean contains(Object obj) {return false;}
        public E get(int index) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
    }
}

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    public boolean add(E e) {
        add(size(), e);
        return true;
    }
    
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }
}

我们可以发现emptyList最后执行的是AbstractList里面的add方法，所以会直接抛出异常。为了避免报错，有同事提议将emptyList都用new ArrayList()代替，此时决定看下emptyList的优势

代码

通过百度知道emptyList不需要占用内存，而ArrayList每次new都会在堆中开辟内存空间存放对象，我们先通过代码验证一下

public class ListTest {
    private static final int printCount = 10000;

    public static void main(String[] args) {
       
        long freeMemory = Runtime.getRuntime().freeMemory();
        System.out.println("freeMemory: " + freeMemory);

        for (int i = 0; i < printCount; i++) {
            List newList = new ArrayList();
        }

        long freeMemoryNew=Runtime.getRuntime().freeMemory();
        System.out.println("freeMemory use: "+(freeMemory-freeMemoryNew));

        for(int i = 0;i < printCount; i++){
            List emptyList = Collections.emptyList();
        }
        long freeMemoryEmpty = Runtime.getRuntime().freeMemory();
        System.out.println("freeMemory use: "+(freeMemoryNew-freeMemoryEmpty));
    }
}

此时我们看一下执行结果

Connected to the target VM, address: '127.0.0.1:63534', transport: 'socket'
Disconnected from the target VM, address: '127.0.0.1:63534', transport: 'socket'
freeMemory: 253398816
freeMemory use: 1430376
freeMemory use: 0

Process finished with exit code 0

我们可以看出new ArrayList执行一万次会消耗1430376KB内存，而Collections.emptyList不会消耗内存，那有人会说emptyList不也是new EmptyList()吗？其实我们再仔细看下上面的源码就发现emptyList是一个static变量，只会初始化一次，所以后续使用不会再初始化对象。此时我们可以得出结论，emptyList不占用内存，但是无法执行add等方法，new ArrayList()占用内存，但是会初始化对象数组，可以执行add等方法。