Java中的集合（面试题）

public class HashMapTest {
	public static void main(String[] args) {
		HashMap<Integer, User> users = new HashMap<>() ;
		users.put(1, new User ("张三"，25) ) ;
		users.put(3, new User ("李四"，22) );
		users.put(2, new User ("王五"，28) ) ;
		System.out.println(users) ;
		HashMap<Integer, User> sortHashMap = sortHas hMap (users) ;
		System. out . print ln (sortHashMap) ;
		/**
		*控制台输出内容
		* {1=User [name=张三， age=25]，2=User [name=王五， age=28]， 3=User [name=李四， age=22] }
		  {2=User [name=王五， age=28]， 1=User [name=张三， age=25]， 3-User [name=李四， age=22]}
		*/
	}
	public static HashMap<Integer, User> sortHashMap (HashMap<Integer, User> map) {
		//首先拿到map的键值对集合
		Set<Entry<Integer, User>> entrySet = map.entrySet() ;
		// 将set 集合转为List集合，为什么，为了使用工具类的排序方法
		List<Entry<Integer, User>> list = new ArrayList<Entry<Integer, User>> (entrySet);
		//使用Collections集合工具类对list进行排序，排序规则使用匿名内部类来实现
		Collections.sort (list, new Comparator<Entry<Integer, User>>() {
		@Override
		public int compare (Entry<Integer, User> o1, Entry<Integer, User> o2) {
		//按照要求根据User的age的倒序进行排
		return o2.getValue().getAge()-o1.getValue().getAge() ;
		}) ;
		//创建-一个新的有序的HashMap子类的集合
		Linke dHashMap<Integer, User> linkedHashMap = new LinkedHashMap<Integer, User> () ;
		//将List中的数据存储在LinkedHashMap中
		for (Entry<Integer, User> entry : list) {
		linkedHashMap.put (entry.getKey(), entry.getValue()) ;
		}
		//返回结果
		return linkedHashMap;
	}
}

//Collections. synch ronizedCollection (c )
//Collections. synch ronizedList (list)
//Collec tions. synch roni ze dMap (m)
//Collections. synchronizedSet (s)

/**
* Constructs a new {@code ArrayList} instance with zero initial capacity.
* /
public ArrayList() {
array = EmptyArray.OBJECT;

public final class EmptyArray {
	private EmptyArray() {}
	public static final boolean[] BOOLEAN = new boolean[0] ;
	public static final byte[] BYTE = new byte[0] ;
	public static final char[] CHAR = new char[0] ;
	public static final double[] DOUBLE = new double[0] ;
	public static final int[] INT = new int[0] ;
	public static final Class<?>[] CLASS = new Class[0] ;
	public static final Object[] OBJECT = new Object[0] ;
	public static final String[] STRING = new String[0] ;
	public static final Throwable[] THROWABLE = new Throwable[0] ;
	public static final StackTraceElement[] STACK_TRACE_ELEMENT = new StackTraceElement[0];
}

/**
* Constructs a new instance of {@code ArrayList} with the specified
* initial capacity.
* @param capacity
*	 the initial capacity of this {@code ArrayList} .
*/
public ArrayList(int capacity) {
	if (capacity < 0) {
	throw new IllegalArgumentException ("capacity < 0: " + capacity) ;
	}
	array = (capacity == 0 ? EmptyArray.OBJECT:new object [capacity]);
}

/**
* Constructs a new instance of {@code ArrayList} containing the elements of
* the specified collection.
* @param collection
* the collection of elements to add.
*/
public ArrayList (Collection<? extends E> collection) {
	if (collection == null) {
		throw new Nul1PointerException("collection == null");
	}
	object[] a = collection.toArray() ;
	if(a.getClass()!= object[].class) {
		object[] newArray = new object[a.length];
		System.arraycopy(a,0,newArray,0, a.length) ;
		a = newArray;
	}
	array = a;
	size = a.length;
}

/*
* Adds the specified object at the end of this {@code ArrayList}.
* @param obj ect
* the object to add.
* @return always true
*/
@Override public boolean add(E object) {
	object[] a = array;
	int s = size;
	if (s == a.length){
		object[] newArray = new object[s +
		(s < (MIN CAPACITY_ INCREMENT / 2) ?
		MIN CAPACITY INCREMENT : s >> 1)];
		System. arraycopy(a, 0, newArray, 0, s);
		array= a= newArray;
	}
	a[s] = object;
	size = s+1;
	modCount++;
	return true;
}

/*Removes the object at the specified location from this list.
* @param index
* the index of the object to remove 。
* areturn the removed object. 
* @throws IndexOu tofBounds Exception
* when {@code location < 0 || location >= size() }
*/
@Override public E remove (int index) {
	object[] a = array;
	int s = size;
	if (index >= s) {
		throwIndexOu tOfBounds Exception (index, s);
	}
	@SuppressWarnings ("unchecked")
	E result = (E) a [index] ;
	System.arraycopy(a, index + 1, a, index, --s -index) ;
	a[s] = null; // Prevent memory leak
	size = s
	modCount++;
	return result;
}

System.arraycopy(a, index + 1，a, index, --s - index);

/**
* Removes all elements from this {@code ArrayList. leaving it * * ewpty.
* @see #isEmpty
* @see #size
*/
Override public void clear() {
	if (size != 0){
		Arrays.fil(array.0, size,null);
		size = 0;
		modCount++;
	}
}

public class Test {
	public static void main (String[] args) {
		Map<String, String> map = new HashMap<String,String>();// HashMap对象
		Map<String, String> tableMap = new Hashtable<String, String>(); //HashTable对象
		map.put(null, null) ;
		System.out.println ("hashMap的[key]和[value ]均可以为null:"+map.get(null));
		try {
			tableMap.put(null,"3");
			System.out.println(tableMap.get(null));
		} catch (Exception e) {
			System.out.println(" [ERROR] : hashTable的[key]不能为null") ;
		}
		try {
			tableMap.put ("3",null);
			System.out.println(tableMap.get ("3"));
		} catch(Exception e) {
			System.out.println(" [ERROR] : hashTable的[value]不能为null");
		}
	}
}