Java Basics

1. Object & Variables

1.1 Object

Everything in Java is Object

1.2 Variables

是一个被name命名的storage location，里面存储着一定的value。
可以具有不同的types

2. Control Structures

To modify the flow of code in a program

2.1 condition

2.2 loop

for
while
foreach

for(char c : charArray) {
  ...
}

3. Data Structures

To store data in an organized and efficient manner

3.1 Array

fixed size

char[] charArray = new char[5];

3.2 ArrayList

flexible size

List<Character> arrayList = new ArrayList<Character>();

3.3 Maps

Key : Value

Map<String, List<String>> map = new HashMap<String, List<String>>();

4. Primitives & Wrapper

4.1 Primitives & its Object counterpart(Wrapper)

int & Integer
boolean & Boolean
double & Double
float & Float
char & Character

4.2 Primitives Size & Default value

8种原始数据类型

Primitive Data Type
Wrapper的default value为null

4.3 什么时候使用Primitive，什么时候使用wrapper？

Primitive更efficient，当需要参与的计算非常的直接，不需要处理类型的转换，不需要考虑极大，极小值等等时，优先考虑Primitive。反之，考虑使用Wrapper。

5. Methods

code reuse

5.1 Access Control

a.k.a Access Modifiers

Visible to the package, the default. No modifiers are needed.
Visible to the class only (private).
Visible to the world (public).

because of class inheritance, all public methods and variables of a class are inherited by its subclasses. 所有public的东西会被子类继承

Visible to the package and all subclasses (protected).

同一个包，或者子类(无论同不同包)。因此可以理解为，隔离了非子类且不同包

5.2 Return Types

好处：在函数定义的时候指明返回值类型，可以使对函数的使用更加明确。