Map也是属于Collection Framework，但它在hierarchy中是独立的一支。
Collection：单值。
Map：键-值对,no duplicated keys and at most 1 value.
HashMap?
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类，实现Map接口。
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HashMap可以用null来做key和value。
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Not thread-safe as non synchronized。
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Why hashCode &equals methods are important and should be overridden?
通过计算key的hashCode来find someplace in the hash buckets to store the key-value entry,and LinkedList is used to store the collided keys entry in each bucket.
And equals是用来fetch the correct entry when collision is met in the same bucket.
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好的key应该具备?
immutable(caching keys), final, with proper defined hashCode and equals methods.
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HashMap有相应的capacity和load factor，当容量满时，会扩容并rehashing.
Hashtable?
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类，实现Map接口，同HashMap是一个层次的。
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Hashtable不接受null。
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Thread-safe。
HashSet?
- HashSet uses HashMap internally.Objects are stored as keys and dummy Object as values.
Differences?
|
| null? | synchronized? |
| HashMap | y | n |
| Hashtable | n | y |
| HashSet | y | n |