Java object-oriented knowledge points sorting out

1. Basic data types (four categories and eight types)

Integer type: byte short int long

Floating point type: float double

Character type: char (' a')

Boolean type: boolean

(Learning video recommendation: java course)

For Object

1. Object: a specific thing that exists in real life

2. Class: the extraction of common attributes and behaviors of something

3. Members The difference between variables and local variables:

1. Application scope

Member variables: valid in the entire class

Local variables: valid within the method in which they are declared

Summary: The application scope of the variable is valid within the {} where it is located

2. Life cycle

Member variables: belong to the object, are created with the creation of the object, and disappear with the object To disappear, you don’t need to assign an initial value and have a default value.

Local variables: Use them to release space immediately. After the definition is completed, an initial value must be assigned.

3. Memory address

Member variables: heap memory

Local variables: stack memory

4. Benefits of encapsulation

Hidden The implementation process of the class is simplified

Simple operation

Improves the security of object data

5. The role of this

1. Solve the problem of member variables and local Problems with variables with the same name

2. Simplify code writing

3. Call other constructors in the constructor. (this must be the first sentence)

6: Overview of static keyword

The static keyword can be used to modify member variables, methods and code blocks of a class.

The variables modified by static are called static variables.

The method modified by static is called a static method.

The initialization code block modified by static is called a static initialization block.

static modified class, static internal class

7. Characteristics of static variables

Static explanation: static keyword

Static, static. Static variables will not change as the object changes. Loading timing:

Loads as the class is loaded.

As the class is loaded into the method area, the static variables directly open up the memory space for storing static variables in the static area.

Static variables exist prior to objects.

Static variables are shared by all objects of the class at the code level: they can be called directly using the class name, without using the object name. This static variable can still be used without creating an object. It is recommended to use class name to access.

8. The difference between static variables and instance variables

Storage location

1. Class variables are loaded as the class is loaded and exist in the method area

2. Instance variables exist in the heap memory as the object is created

Life cycle

1. Class variables have the longest life cycle and disappear when the class disappears

2. The life cycle of instance variables disappears when the object disappears

9. Static advantages and disadvantages

1. Advantages: Store the shared data of the object in a separate space to save space

2. Disadvantages: The life cycle is too long

There are limitations in access. (Static can only access static)

10. The difference between StringBuffer and StringBuilder

Same points:

Both are string buffers and string generators , are all variable character sequences

Differences:

1. Different thread safety:

StringBuffer is thread-safe and data is still guaranteed in a multi-threaded environment Security

StringBuilder is thread-unsafe and cannot guarantee data security in a multi-threaded environment

2. Different efficiency:

StringBuffer is inefficient

StringBuilder efficiency High

11. Abstract method

A method modified with the abstract keyword is called an abstract method.

Characteristics of abstract methods: only method declaration, no method body.

The role of abstract methods: overridden by subclasses, stipulates the method calling format for subclasses, and the specific operation content waits for subclasses to complete.

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