Java Declaration 5 Min Drill
- Arrays are objects in Java.
- In Java an array is a collection of ordered
primitives, object references.
- It is never legal to include the size of an array
in the declaration.
- An array of objects can hold any object that passes
the IS-A (or instanceof) test for the declared type of the array.
e.g, if Circle extends Shape, then a Circle object can go into an
- A class is usually described as the template or
blueprint from which the object is actually made.
- A class can also be defined as new data type.
- An instance of a class is an object.
- In Java the methods are similar to functions in
- An abstract classes cannot be instantiated. If you
define one method as abstract in a class the class must be defined
- Abstract methods does not have method
implementation (i.e. method bodies), the implementation is provided
by the subclasses.
- In Java constructors are used to initialize the
object. The name of the constructor is the same name as the class
- If you define one non-default constructor in any
given class, then the default constructor will not be provided by
- A source code file can have only one public class.
- If the source file contains a public class, the
filename must match the public class name.
- A file can have only one package statement, but it
can have multiple imports.
- The package statement (if any) must be the first
(noncomment) line in a source file.
- The import statements (if any) must come after the
package and before the class declaration.
- If there is no package statement, import statements
must be the first (noncomment) statements in the source file.
- A file can have more than one nonpublic class.
- Files with no public classes have no naming
- An interface can have only abstract methods, no
concrete methods allowed (upto Java 1.7).
- Interface methods are by default public and
abstract explicit declaration of these modifiers is optional.
- Interfaces can have constants, which are always
implicitly public, static, and final.
- Interface constant declarations of public, static,
and final are optional in any combination.
- Class implementing an interface can itself be
- An abstract implementing class does not have to
implement the interface methods (but the first concrete subclass
- Class can extend only one class, but it can
implement many interfaces.
- Interfaces can extend one or more other interfaces.
- A subclass can inherit a member of its superclass.
- this. always refers to the currently executing
- The BigDecimal is immutable
BigDecimal test = new BigDecimal(0);
System.out.println(test); //Result 0
test = test.add(new BigDecimal(30));
System.out.println(test); //Result 30