Generics, Lambdas and Reflection SPL – PS5 Generics, Lambdas and Reflection

Overview Return to Java Generics Anonymous classes and lambdas Default methods Reflection, class object

Java collections Collections are objects that holds other objects in some manner. For example: List, Map, and Set. Java platform provides many ready to use collection classes. You should usually work with Java’s collection classes, instead of arrays.

Generics A mechanism for creating objects parameterized by a type. For example – a stack. We want to create a stack of integers And a stack of cows

Generics (cont) Since there is code duplication, we may want to create a generalized stack. This stack can hold both cows and integers at the same time! Usually that’s not what we want. The solution – Use generics

Generics example

Default methods A default method is a method which is implemented directly in the interface. There’s a straight-forward way to implement the method isEmpty()

Default methods(cont) This implementation can suite most implementations of stack. We can add this implementation to the interface using default methods.

Generic methods In addition to generic classes and interfaces, Java supports generic methods. Let’s assume this is our Cow class: How do we define the maximal object in an array of cows?

Reminder: Comparator Given two objects of type T, the method compare will compare them and return: A positive integer if o1>o2 A negative integer if o2>o1 Zero if both objects are equal

Generic methods(cont) The following is a generic method used to find a maximal element in an array.

Generic methods(cont)

Anonymous classes Anonymous classes help make you your code more concise. Enable you to declare and instantiate a class at the same time. Use them if you need to use the class only once.

Lambda expressions An issue with anonymous classes is that their syntax can be a bit unclear. When the class we want to instantiate has only one method, we can use lambda expressions.

Lambda expressions (cont) The lambda syntax is (args…) -> {body} You can replace an anonymous class with a lambda expression only if the class has a single non-default method. If all your lambda does is return a value, you can replace (args…) -> {return ans;} with (args…) -> ans; No need to declare the type of the parameter No need to declare a single parameter in parenthesis (Required for multiple parameters) If lambda’s body contain a single expression, no need for curly braces.

Lambda expression example

Class object There’s a special class in Java called Class. Instances of class Class represents classes and instances running in the Java application. Every class in Java has a corresponding Class object. You can receive the object Class in runtime using the static class property, or the method getClass(). The class Object has several methods, getName() which returns the name of the class, and isAssignableFrom(class) which returns true if the calling class is a super-class or interface of the given class.

Class object example

Design by contract DBC is a simple terminology that helps us express contracts between specification and implementation. DBC relies on three words: Precondition – things that must be true before a method is invoked. Postcondition – things that must be true after a method is invoked. Invariants – things that must be true both before and after any method is invoked, and immediately after construction.

Rules that ensure testability Separate commands and queries. Separate basic queries and derived queries. Define derived queries in terms of basic queries. For each basic command, write post-condition which specify the value of every basic query. For each basic command and basic query, express the pre-conditions in terms of basic queries. Specify class invariants that impose constraints that must remain always true on basic queries.