I. Objective-C Basic Grammar

• 1. String
  The string of Objective-C is wrapped in double quotation marks and preceded by an @ symbol.

  title = @"Hello World";
  if(title == @"hello World") {}
  

• 2. Function Call

  • No parameters

    justRun();
    

  • With parameters

    NSLog(@"show string: %@", str);
    CGRectMake(0, 0, 0, 0);
    

• 3. Passing messages to class/instance methods (sending messages to objects)

  • No parameters

    [obj method];
    

  Corresponding java version
  java obj.method();

  • With one parameter:

    [car fly:1];
    

  Corresponding java version
  car fly(1);

  • With multiple parameters
    The non-first parameter should have a similar key-value pair to represent the parameter Blue:(float)blue, that is, the method names are stitched together.

    - (void) setColorToRed: (float)red Green: (float)green Blue:(float)blue {...} //Definition method
    [myColor setColorToRed: 1.0 Green: 0.8 Blue: 0.2]; //Call method
    

    Corresponding java version

    public void setColorToRedGreenBlue(float red, float green, float blue) {...}
    myObj.setColorToRedGreenBlue(1.0, 0.8, 0.2);  
    

• 4. Function Chain Call

  UINavigationBar *bar = [[[UINavigationBar alloc] init] autorelease];
  

Corresponding Java code
java UINavigationBar bar = UINavigationBar.alloc().init().autorelease();

• 5. Creating Objects

Creating objects requires two messages, alloc and init. Alloc allocates memory and init initializes objects
objectivec MyObject * my = [[MyObject alloc] init];
In Objective-C 2.0, you can use new directly if you don't need parameters to create objects.
objectivec MyObject * my = [MyObject new];

To define the initialization process yourself, you can override the init method to add additional work. (constructor similar to C++)
objectivec - (id) init {if (self= [super init]) {// must call init // do something here...} return self;}

• 6. id dynamic type

id is a generic object type that can point to objects belonging to any class
objectivec id obj = [Person new];

II. Categories

Object-C classes are divided into two parts: interface definition and implementation. Interfaces and implementations begin with @interface, @implementation, and end with @end. The symbol @ is a magical symbol in Objective-C.

• 1. Interface Definition

Interface is placed in the header file with the file extension. h

Object-C Interface is more like a Class class in java
The Protocol of Objective-C is like the Interface interface in java

Object-C Customization An Interface Example

  @interface MyObject {
       int memberVar1;
       id  memberVar2;
   }

  -(void) instance_method1;
  -(BOOL) instance_method2: (int) p1;
  -(UITableViewCell *) instance_method3: (int) p1 andPar: (int) p2;
  @end

• 2. implementation of Interface

In the implementation file, the file extension is.m

```objectivec
 @implementation MyObject {
     int memberVar3;
 }

-(void) instance_method1 {
    ....
}
-(BOOL) instance_method2: (int) p1 {
    ....
}
 -(UITableViewCell *) instance_method3: (int) p1 andPar: (int) p2 {
     ....
 }
@end
```

Java version of the above code

```Java
public class MyObject {
    protected int memberVar1;
    protected OtherClass memberVar2;
    private int memberVar3;

    public void instance_method1() {
        ....
    }

    public boolean instance_method2(int p1) {
        ....
    }

    public UITableViewCell instance_method3andPar(int p1, int p2) {
        ....
    }
}
```

• 3. Class static method and instance method

  Static methods: Class methods have a plus prefix
  Instance method: Instance method has a minus prefix

  Class Definition

  @interface MyObject : NSObject
      +(void) sayHello;
      -(void) sayHello2;
  @end
  
  @implementation MyObject
  
  +(void) sayHello {
      NSLog(@"Hello, World");
  }
  -(void) sayHello2 {
      NSLog(@"Hello, World2");
  }
  @end
  

  call

  [MyClass sayHello];
  
  MyObject *mycls = [MyObject new];
  [mycls sayHello2];
  

• 4. Class Inheritance

  @interface MyObject : NSObject
  @end
  

  Corresponding java code

  public class MyObject extends NSObject {
  }
  

Protocol

Interface interface equivalent to Java

• 1. Definition of protocol

  @protocol Locking
      -(void)lock:(NSString)str;
  @end
  

  Corresponding Java code

  publilc interface Locking {
      public void lock(String str);
  }
  

• 2. Inheritance of Agreement

  The agreement itself can also inherit other agreements.

  @protocol Locking <NSObject>
      -(void)lock:(NSString)str;
  @end
  

  Corresponding Java code

  public interface Locking extends NSObject {
      public void lock (String str);
  }
  

• 3. Optional methods

The protocol can contain optional methods, which, as the name implies, can be implemented without classes, with the @optional keyword added, and a class can not implement print:methods when implements the protocol.
objectivec @protocol Locking @optional -(void)lock:(NSString)str; @end

• 4. Implementation of Protocol

A class implements certain protocols that are written in the Interface definition, with multiple protocol names separated by commas
```objectivec

@interface  MyLockingController : NSObject <Locking, Drawable>

@end
```

Corresponding Java code

public class MyLockingController extends NSObject implements Locking, Drawable {
}

objectivec Reference Link on Wikipedia