泛型

23.不要在新代码中使用原生态类型

24.消除非受检警告

25.列表优于数组

数组是协变的

public static void main(String[] args) {Object [] objects = new Long[1];objects[0] = "1";System.out.println(objects[0]);}

只有在运行时才会报错

数组是具体化的，因此数组会在运行时才知道并坚持它们的元素类型约束，泛型是通过擦除来实现的，因此泛型只在编译时强化类型信息，并在运行时丢弃元素类型信息，擦除就是使泛型可以和每页使用泛型的代码随意进行互换

26.优先考虑泛型

public class Stack {private Object[] elements;private int size = 0;private static final int DEFAULT_INITIAL_CAPACITY = 16;public Stack(){elements = new Object[DEFAULT_INITIAL_CAPACITY];}public void push(Object e){ensureCapacity();elements[size++] = e;}public Object pop(){if(size == 0){throw new EmptyStackException();}Object result = elements[--size];elements [size] =null;return result;}public boolean isEmpty(){return size ==0;}private void ensureCapacity() {if(elements.length ==size){elements =Arrays.copyOf(elements, 2*size+1);}}}

泛型化

public class Stack<E> {private Object[] elements;private int size = 0;private static final int DEFAULT_INITIAL_CAPACITY = 16;@SuppressWarnings("unchecked")public Stack(){//方法一//elements = (E[]) new Object[DEFAULT_INITIAL_CAPACITY];//方法二elements = new Object[DEFAULT_INITIAL_CAPACITY];}public void push(E e){ensureCapacity();elements[size++] = e;}public E pop(){if(size == 0){throw new EmptyStackException();}//方法2E result =(E)elements[--size];elements [size] =null;return result;}public boolean isEmpty(){return size ==0;}private void ensureCapacity() {if(elements.length ==size){elements =Arrays.copyOf(elements, 2*size+1);}}} 

27.优先使用泛型方法

//有警告public static Set union(Set s1,Set s2){Set result = new HashSet(s1);result.addAll(s2);return result;}//修正，使方法变成类型安全的public static <E> Set<E> union(Set<E> s1,Set<E> s2){Set<E> result = new HashSet<>(s1);result.addAll(s2);return result;}

泛型方法无需明确类型参数的值，不像调用泛型构造器那样必须指定，编译器通过检查方法参数的类型来计算类型参数的值，类型推导

泛型构造器，明确传递参数的值，类型参数出现在变量声明的俩边，冗余，可以使用泛型静态工厂

Map<String, List<String>> anagarms = new HashMap<String ,List<String>>();//泛型静态工厂方法public static<K,V> HashMap<K, V> newHashMap(){return new HashMap<K,V>();}Map<String, List<String>> anagarms =newHashMap();//现在可以Map<String, List<String>> anagarms = new HashMap<>();

28.利用有限制通配符来提升API的灵活性

public void pushAll(Iterator<E> src){for (E e:src) {push(e);}}

Stack<Number> numberStack = new Stack<>();Iterator<Integer> iterator =;numberStack.pushAll(integers);

如果泛型不一样会报错
可以用有限制的通配符来处理

Iterator<? Extends E>,E的某个子类型迭代器接口

public void pushAll(Iterator<? extends E> src){for (E e:src) {push(e);}}

public void popAll(Collection<? extends E>){while (!isEmpty) {dst.add(pop());}}

类型匹配不严格，使用通配符类型，严格就没必要使用通配符
生产者<? extends T>,消费者<? super T>

29.