【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