About Java

1.

Getting straight to the point, Java's parameter passing method is 'Pass by Value with Shallow Copy'.

For example,

Let's 

And I guess this make sense for Java because Java does not support copy constructor. If it were C++, code like above should have been supposed as an error, especially because of not defining copy constructor. Pass-by-Value is not supposed to manipulate any data of the original instance. 

However, there is no copy constructor on Java and everybody's using the parameter passing as if it were 'Pass-by-Value for primitive types, but Pass-by-Reference for reference types.'

2.
Wrapper class is a class that is used to wrap primitive type(int, float, long... etc) as a 'class'. It is usually a name of its primitive type starting with a larger case: int -> Integer, float -> Float

It is used to replace variable that contains primitive type with a instance of a class. It is also used to describe a 'NULL' state because primitive type cannot.

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Interface of Java

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Leetcode

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1. Two Sum: find a pair with specific sum   class Solution : def twoSum ( self , nums : List [ int ] , target : int ) - > List [ int ] : numMap = { } n = len ( nums ) # Build the hash table for i in range ( n ) : numMap [ nums [ i ] ] = i # Find the complement for i in range ( n ) : complement = target - nums [ i ] if complement in numMap and numMap [ complement ] != i : return [ i , numMap [ complement ] ] return [ ] # No solution found You might wonder how does the code deals with duplicates. As you can see, in the iteration of building the hash table, always the latter(with bigger index) will be saved in the hash table. In the latter iteration finding the complement, the former one(with smaller index) always goes through the iteration earlier, so the [i, numMap[complement]] can return two different indexes, carrying duplicate...
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