Default methods were introduced in interfaces to provide new functionality while maintaining backwards compatibility for existing implementations. Default methods allow interfaces to define implementations for methods while classes implementing those interfaces can also define their own implementations or use the default. Resolution rules for determining which implementation to use are that classes always win over interfaces, sub-interfaces win over parent interfaces, and ambiguous cases result in errors.

1. Default Methods

2. Default Methods
 Why do we need default methods ?
 Usage
 Abstract Classes vs Interface with default methods

3. Why do we need default methods ?
Public void apply()
Public void apply(){}
Public void apply(){}
Public void apply(){}
Interface-v0
Class
Class
Class
Public void apply()
Public void execute()
Public void apply(){}
Public void execute(){}
Public void apply(){}
Public void execute(){}
Public void apply(){}
Public void execute(){}
Interface-v1
Class
Class
Class

4. Usage

5. Resolution Rules
 Classes always win
Default void apply(){}
void apply(){}
Interface A
Class B implements A
New c().apply()

6. Resolution Rules
 sub-interfaces wins
Default void apply(){}
Interface A
Class C implements B
Default void apply(){}
Interface B extends ANew c().apply()

7. Resolution Rules
 Ambiguous
Default void apply(){}
Interface A
Class C implements A,B
Default void apply(){}
Interface B
New c().apply()