Hashed collections哈希集合

  【定义】

有index的集合

 

【hash的原理】

term for a situation when two different objects return the same hashcode： hash collision

就是无规律的一一对应排序，相同object对应的HASH应该相同，相同对应的HASH应该不同。具体实现：hashCode(It returns an int hash-code depending on the memory address of the object 根据obeject的位置来对应in the heap) equal() 都是自动继承的。

Use a function on hashcode, such as modulo, to calculate the index into the hashtable and then store the object at that index 根据hash的index来查找物体

 

【实现】

object有三种方法toString equal hashcode，通过传入memory address来计算值。同一obeject的不同state状态：要避免collision。

 

public int hashCode() {  
return  Objects.hash(title, author, year);   //hash on instance variable values
}

//equals() implements equivalence relation - reflexive, symmetric, transitive, consistent – 
// on non-null object references
@Override
public boolean equals(Object o) { 
if (o == null) return false;  // if o is null, then it can’t be equal to ‘this’ 
if (this == o) return true;   // if both point to same object, then they are equal
if (getClass() != o.getClass()) return false; //if not of same type, they can’t be equal
Book b = (Book) o; //now that we know o is unique, non-null Book object, cast it to Book
return title.equals(b.title) && author.equals(b.author) && (year == b.year); //compare values
}

【步骤】

先用hashcode，再用equals。因为state不同要overload, 只overload一个会发生inconsistent。

 

【区别】

用object/key来hash

 

【treemap】

排序的map，里面是BST

【map的三个集合】

entryset keyset values 

remove(k) 是remove key = k的entry