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Java Generics
Summary - Java Generics
Introduction to Generics in Java
Generics in Java enable types (classes and interfaces) to be parameters when defining classes, interfaces, and methods. They provide a way to reuse the same code with different data types while ensuring type safety.
Key Concepts of Generics:
1. Generics
Generics allow you to define classes, interfaces, and methods with a placeholder for the type they operate on. This placeholder is replaced with a specific type when the code is executed.
Benefits:
2. Type Safety
Type safety ensures that a variable is only assigned values of a specific type. Generics provide compile-time type checking, which helps catch errors early in the development process.
Example Without Generics:
Example With Generics:
3. Type Parameter
A type parameter is a placeholder for a type that is specified when a generic class, interface, or method is instantiated. It is typically represented by a single uppercase letter, such as T, E, K, or V.
4. Generic Class
A generic class can operate on objects of various types while providing compile-time type safety.
5. Generic Methods
A generic method can operate on objects of various types. The type parameter is specified immediately before the return type.
6. Wrapper Classes Instead of Primitives
Java generics work with reference types, not primitive types. Wrapper classes such as Integer, Double, Character, and Boolean are used to represent primitive data types.
7. Raw Types
A raw type is the name of a generic class or interface without its type parameters. Raw types are allowed for backward compatibility, but they should be avoided because they lose the benefits of type safety.
Bounded Types in Java
Bounded types in Java enable you to restrict the types that can be used as type arguments in generics. This allows for more flexible and reusable code while maintaining type safety.
Key Concepts of Bounded Types:
1. Bounded Types
Bounded types in generics allow you to specify a range of types that can be used as type arguments using the extends keyword for upper bounds and the super keyword for lower bounds.
2. Upper Bound
An upper bound restricts the type parameter to be a specific type or a subtype of that type. It is specified using the extends keyword.
3. Lower Bound
A lower bound restricts the type parameter to be a specific type or a supertype of that type. It is specified using the super keyword.
4. Multiple Bounds
Multiple bounds allow you to specify more than one constraint on a type parameter. The type parameter must satisfy all specified bounds, using the & symbol.
5. Upper Bound on Class Level
An upper bound can be applied to a type parameter at the class level, restricting the type that can be used when the class is instantiated.
6. Upper Bound on Method Level
An upper bound can be applied to a type parameter at the method level, restricting the type that can be used when the method is called.
7. Lower Bound on Method Level
A lower bound can be applied to a type parameter at the method level, indicating that the type must be a specific type or a supertype of that type.
Wildcards in Java
Wildcards in Java are used to represent an unknown type. They are particularly useful in the context of generics, where they allow for more flexible and reusable code. Wildcards can be unbounded, upper-bounded, or lower-bounded.
Key Concepts of Wildcards:
1. Wildcards
A wildcard in Java is represented by the ? symbol and is used to denote an unknown type in generics.
2. Unbounded Wildcards
An unbounded wildcard is represented by the ? symbol without any bounds. It can be used when the type is unknown and no restrictions are needed on the type.
3. Upper Bounded Wildcards
An upper bounded wildcard restricts the unknown type to be a specific type or a subtype of that type. It is represented by the ? extends Type syntax.
4. Lower Bounded Wildcards
A lower bounded wildcard restricts the unknown type to be a specific type or a supertype of that type. It is represented by the ? super Type syntax.
5. Restrictions of Wildcards
Wildcards have certain restrictions to ensure type safety and prevent runtime errors:
Type Erasure in Java
Type erasure is a process that occurs during the compilation of Java code that uses generics. It removes all generic type information, replacing it with the appropriate bounds or Object if no bounds are specified. This allows for backward compatibility with legacy code that does not use generics.
Key Concepts of Type Erasure:
1. Type Erasure
Type erasure is the process by which generic type information is removed during compilation. Generic type parameters are replaced with their bounds or Object if no bounds are specified.
2. Why Type Erasure Was Introduced?
Type erasure was introduced to maintain backward compatibility with older versions of Java that did not support generics. It allows new code that uses generics to interoperate with legacy code that uses raw types.
3. Backward Compatibility with Legacy Code
Backward compatibility ensures that new features in the language, such as generics, do not break existing code. Type erasure allows generic and non-generic code to coexist and work together.
4. Generic Types Changes After Type Erasure
During type erasure, generic type parameters are replaced with their bounds or Object if no bounds are specified.
5. Ambiguity Errors Due to Type Erasure
Type erasure can cause ambiguity errors when methods that were distinct due to their generic types become identical after erasure, causing compile-time errors due to the ambiguity.
By understanding Java Generics, Java developers can write generic code compatible with legacy code and avoid common pitfalls such as ambiguity errors. For more detailed information, refer to the Notion link:
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