Preface :Java We can't stop our basic knowledge .


1.Array and ArrayList The difference between

About Array Usage of , see :http://blog.csdn.net/b_11111/article/details/53573742

Array by java.lang.reflect A class under the package , Provides a way to dynamically create and access arrays . Can contain basic data types and object types . Fixed size , Because the length of the array is determined when it is created .

ArrayList yes List Interface implementation . Can only contain object types , Size can change dynamically , Elements can be null. Default size ( Capacity ) by 10. Thread unsafe .

Add capacity :int newCapacity = oldCapacity + (oldCapacity >> 1); Equivalent to expansion 1.5 times .

ArrayList Source analysis portal :ArrayList Source code analysis

2.HashSet and LinkedHashSet

HashSet Implementation principle of , see :http://zhangshixi.iteye.com/blog/673143

HashSet The main characteristics of :

1)HashSet Inherit AbstractSet, Realization Set Interface .

2) disorder , allow null value .

3) be based on HashMap The implementation of the . So threads are not safe .

LinkedHashSet Implementation principle of , see :http://zhangshixi.iteye.com/blog/673319

LinkedHashSet The main characteristics of :

1)LinkedHashSet Inherit HashSet, Realization Set Interface .

2) Orderly , Insertion order , allow null value , Because inheritance HashSet.

3) Thread unsafe .

3.HashMap And LinkedHashMap

HashMap stay Java Basic knowledge points ( One ) It has been introduced in , Here we mainly introduce LinkedHashMap.

LinkedHashMap Implementation principle of , see :http://zhangshixi.iteye.com/blog/673789

LinkedHashMap The main characteristics of :

1)LinkedHashMap Inherit HashMap, Realization Map Interface .

2) Orderly , Insertion order , allow null value .

3) Thread unsafe .

3. Boxing and UnBoxing

About packing and unpacking , Specific details , see :http://www.cnblogs.com/dolphin0520/p/3780005.html

The main points of :

from jdk1.5 Start , It provides the function of automatic packing . Such as Integer i=10, This statement automatically creates the corresponding Integer object , That's packing .

And unpacking , On the contrary , The following code :

Integer i=10;// Packing

int x=i;// Unpacking

in short , Boxing is the automatic conversion of basic data types to wrapper types ; Unpacking is automatically converting the wrapper type to the basic data type .

notes :

1) The boxing process is done by calling the wrapper valueOf Method , The unpacking process is done by calling the wrapper xxxValue Method .

2)Integer、Short、Byte、Character、Long These kinds of valueOf The implementation of the method is similar .

When the boxed value is in [-128,127] In the interval , The first thing is to look up this value in the cache . If there is , The value reference is returned directly , If not, create a new object .

When the packing value is not in [-128,127] In the interval , Create a new object directly .

Especially for Character object , Packing operation is , Is to determine whether the value is less than or equal to 127, Not from [-128,127] Interval values .

Double、Float Of valueOf The implementation of the method is similar .

These two types of valueOf Method , When packing , New objects are created directly .( It can be seen directly from the source code that )

Because the number of integer values in a certain range is limited , Floating point numbers are not .

3) Specially pointed out Boolean type , Its valueOf Method :

 public static Boolean valueOf(boolean b) {
return (b ? TRUE : FALSE);
}

among , The above marked TRUE and FALSE For two static variables :

 public static final Boolean TRUE = new Boolean(true);
public static final Boolean FALSE = new Boolean(false);

notes :1) Wrapping objects with basic types equals When the method is used , First, we need to judge whether the types being compared are the same , Without type conversion .

2) For wrapper classes of basic data types , When doing arithmetic , Will carry out automatic unpacking operation , And then calculate the values .

4. Deep copy and shallow copy

About deep copy and shallow copy , see :http://www.cnblogs.com/Jtianlin/p/4605477.html

Deep copy : The object and its associated content are copied .

Shallow copy : For variables of basic data type, a new copy will be made , For variables of reference type, they just copy the reference . So shallow copy modification of one object causes modification of another .

5. Interfaces and abstract classes

About interfaces and abstract classes , Reference resources :http://www.cnblogs.com/dolphin0520/p/3811437.html

Key points :

abstract class :

1) from abstract Modified class , You can't create objects directly , Need to inherit .

2) Not necessarily Abstract ( Abstract methods must be created by public or protected modification , The default is public).

3) You can have common variables and methods , After all, it's a class .

4) It's an abstraction of things .

Interface :

1) from interface modification , It needs to be implemented by class .

2) It can contain variables and methods , But the method has to be abstract , That is to say, there can be no concrete implementation .

3) Variables in the interface are implicitly specified as public static final( And it can only be used public static final modification ), Method is implicitly specified as public abstract Method ( And can only be public abstract Method ).

4) It's an abstraction of behavior .


by Shawn Chen,2018.3.21 Japan , Afternoon .


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