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Free Core Java Online Practice Test and Preparation for Exam
20190110

1.legal identifier

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java naming:Variables
firstLetter: letter ,$,_
number is not allowed in first letter
secondAfter:letter,$,digit,_
+ is not allowed here
20190111
2.variable initialization
local or static.png

Java (https://en.wikipedia.org/wiki/Java_(programming_language) "Java does not have uninitialized variables. Fields of classes and objects that do not have an explicit initializer and elements of arrays are automatically initialized with the default value for their type.
Local variables in Java must be definitely assigned to before they are accessed, or it is a compile error.

3. data type

Primitive Data Types (Java)
Cours java : Types de donnée primitifs

整数类型：
byte:8 bite(-128~127)
short :2 byte = 16 bite
int:4 byte = 32 bite
long:8 byte = 64 bite
浮点型：
float：单精度，32 bite
double: 双精度；64 bite

float is represented in 32 bits, with 1 sign bit, 8 bits of exponent, and 23 bits of the significand.
double is represented in 64 bits, with 1 sign bit, 11 bits of exponent, and 52 bits of significand.
scientific-notation number: 2.33728*1012; 33728 is the significand).

字符型：
char ：2 byte = 16 bite

使用：
byte,short 直接使用
long:long l = 2[L/l];
float:float f = 2.3f or 2.3F;
double:double d = 2.383747[D/d/];
因为long和double在整型和浮点型中范围最大，因此不用刻意指出类型，不会失去精度。
而float如果不指出，possible lossy conversion from double to float

20190115

4 abstract class

abstract class

abstract class may contain abstract method or not, it can't be instantiated like interface, but it can be extended.

20190130
test4

5 bit operations

source:Opérateurs bit à bit

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The Java equivalent of 'unsigned'
Signed vs unsigned(有符号和无符号）
In C:
When an integer is signed, one of its bits becomes the sign bit, meaning that the maximum magnitude of the number is halved. (So an unsigned 32-bit int can store up to 2³²-1, whereas its signed counterpart has a maximum positive value of 2³¹-1.)
In java:

In Java, all integer types are signed (except char).

Signed vs unsigned shifts
this operator performs sign extension: that is, as well as shifting the bits of the number one place to the right, it preserves the sign. Specifically, after performing the shift, it copies the sign bit (the leftmost bit) into the leftmost position.
Now, if we're treating an integer as unsigned, then we don't want to copy the sign bit, because it doesn't actually represent the sign! Instead, we want to leave it as zero. To achieve this, in Java, instead of writing >>, we write >>>.

6.Data range

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byte: 8-bit signed,( -128 , 127).
Short: 16-bit signed,( -32,768 , 32,767).
Int: 32-bit signed,(,-231,231-1)

7 Map vs collection

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Collections Framework Overview

The collection interfaces are divided into two groups.
The most basic interface, java.util.Collection,
The other collection interfaces are based on java.util.Map.

8 overloading

overloading
Defining Methods (The Java Tutorials)

The Java programming language supports overloading methods, and Java can distinguish between methods with different method signatures. This means that methods within a class can have the same name if they have different parameter lists (there are some qualifications to this that will be discussed in the lesson titled "Interfaces and Inheritance").

Attention:parameter lists, not including return type.

9 access level

Controlling Access to Members of a Class (The Java Tutorials)
default vs protected
default = no modifier

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10 Initial Values of Variables

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Chapter 4. Types, Values, and Variables

• Each class variable, instance variable, or array component is initialized with a default value when it is created
• A local variable must be explicitly given a value before it is used, by either initialization or assignment.

11 equals() vs ==

equals vs ==
equals() compare content
== compare memory address

test7

12 if(condition)

condition must be true or false
int a = 1;
if(a) will lead a compile error.

test10