New Style.m

// New Style.

// MyClass.h

@interface MyClass : NSObject
- (void)foo;
@end

// MyClass.m

#import "MyClass.m"

@implementation MyClass

- (void)foo
{
    [self bar];
}

- (void)bar
{
    // Something.
}

@end

Old Style #1.m

// MyClass.m

#import "MyClass.m"

@interface MyClass ()
- (void)bar;
@end

@implementation MyClass

- (void)foo
{
    [self bar];
}

- (void)bar
{
    // Something.
}

@end

Old Style #2.m

// Old style #2.

// MyClass.h

@interface MyClass : NSObject
- (void)foo;
@end

// MyClass.m

#import "MyClass.m"

@implementation MyClass

- (void)bar
{
    // Something.
}

- (void)foo
{
    [self bar];
}

@end