11 [Instructor] Java Essentials ch06

| Class variable, instance variable //property

cv iv

Must be maintained differently for each object iv

Common to all objects cv //static+

Card c = new Card();

c .kind = "HEART"; //iv

c.number = 5;

Card .width = 200; //Cv class name in front, reference variables also work

Card.height = 300;

*Draw according to flash video MemberVar

| What is a method? Method declaration and implementation

Method { } - unit of work

Code duplication needs to be eliminated, easy to manage, reusable, concise

Methods must be in a class, functions must be independent

It is better to write it to perform only one function.

int add (int a, int b) // declaration part, return type int

{ //Implementation part

int result = a + b;

return result;

}

When there is nothing to return, return type void

//Parameter a, b && result → local variable lv

It doesn't matter if the names overlap, as it only applies within the method.

| Method call, method execution flow

Call: method name(); //When calling void print99danAll()

int result = add (3, 5);

class MyMath {

long add (long a, long b) { //(1)

long result = a + b;

return result;

}

long add(long a, long b) { return a + b; } //(2)

//(1) and (2) expressions are the same value

a > b ? a : b;

//If a>b, return a, otherwise b

//Simply used instead of if-else statement, ternary operator

//It can be called without a variable (result) that stores the value, but it cannot be stored in the address, only a call is possible.

1. Object creation

2. Method call

3. All method statements are executed, or return to the calling method in the return statement and execute subsequent statements.

| return statement, return value

void printGugudan(int dan) {

if(!(2

return;

for(int i=1; i

System.out.printf( "%d * %d = %d%n", dan, i, dan*i );

return; // The return type is void, so it can be omitted. Compiler automatically adds

}

//In other cases, it is necessary to write a return statement for both true and false.

//Return type must also match

| call stack call stack

In single thread, only one method is executing, others are waiting.

//Draw step by step

| primitive parameters

Basic type parameter - can only read the value of the variable

Reference parameter - allows you to read and change the value of a variable

*flash video PrimitiveParam, ReferenceParam

public static void main(String[] args) {

Date d = new Date();

d.x = 10; //The x value pointed to by the reference variable d is 10.

System.out.println("main() : x =" +d.x);

change(d.x);

System.out.println("After change(d.x)");

System.out.println("main() : x =" +d.x);

}

static void change( int x ) {

x = 1000;

System.out.println("change() : x =" +x);

}

//The x value of change and x value of Date are different

| reference parameter

public static void main(String[] args) {

Date2 d = new Date2();

d.x = 10;

System.out.println("main() : x =" +d.x);

change(d.x);

System.out.println("After change(d.x)");

System.out.println("main() : x =" +d.x);

}

static void change( Date2 d ) { //Reference type parameter, copy local variable d value of main method to change

d.x = 1000;

System.out.println("change() : x =" +d.x);

}

Reference return type

public static void main(String[] args) {

Data3 d = new Data3();

d.x = 10;

Data3 d2 = copy(d); //No need for reference variable, because it is a static method

System.out.println("d.x ="+d.x);

System.out.println("d2.x"+d2.x);

}

static Data3 copy( Data3 d ) {

Data3 tmp = new Data3();

tmp.x = d.x;

return tmp ; //Returns the address of the reference tmp object

}

//Exchanging the address of the object

*Draw a picture

| static and instant methods

1. static method = class method

Called with ‘class name.method name()’ without creating an object

Instance variables (iv) cannot be used within methods.

Math.ramdom( ), Math.round( )

2. Instant method

After creating an instance, call it with ‘reference variable.method name()’

Instance variables can be used within methods

iv, im

class MyMath2 {

long a, b; //iv instance variable, iv

long add ( ) { //Instance method, no parameters due to use of iv

return a + b;

}

static long add(long a, long b) { //class method (static method), parameters, lv

return a + b;

}

System.out.println(MyMath2.add(200L, 100L); //Class method call, no object (iv bundle) required

MyMath2 mm = new MyMath2 ( ); //Instance method call, object creation required

mm.a = 200;

mm.b = 100;

System.out.println(mm.add( ));

static 1. Attached to a common property 2. Attached to a method that does not use an instance member

Comments