問題描述

public class StrictMap<V> extends HashMap<String, V> { private static final long serialVersionUID = -3455861209780003757L; private String name; public StrictMap(int initialCapacity, float loadFactor, String name) {super(initialCapacity, loadFactor);this.name = name; } public StrictMap(int initialCapacity, String name) {super(initialCapacity);this.name = name; } public StrictMap(String name) {this.name = name; } public String getName() {return name; } public void setName(String name) {this.name = name; } @Override public V put(String key, V value) {if (containsKey(key)) { throw new RuntimeException('已經存在的key, keyName = ' + key);}if (key.contains('.')) { final String shortName = getShortName(key); if (containsKey(shortName)) {// question 這邊有個問題， 為什么沒有拋出ClassCastException。V ambiguity = (V) new Ambiguity('存在的shortName');super.put(shortName, ambiguity); } else {super.put(shortName, value); }}return super.put(key, value); } @Override public V get(Object key) {V value = super.get(key);if (value == null) { throw new RuntimeException('keyName = ' + key + ', 沒有獲取到value.');}if (value instanceof Ambiguity) { throw new RuntimeException('重復的shortName');}return value; } static class Ambiguity {private String name;public Ambiguity(String name) { this.name = name;}public String getName() { return name;} }}

ps:運行時也沒報錯。

問題解答

類型擦除為Object，強轉為Object不會報錯。

編譯時只會有警告，不會報錯的。Java Collections Framework里其實到處是這樣的強轉。但運行時卻會報錯，如果V不是Ambiguity或者它的父類的話。