a Marker interface doesn’t have any methods or fields declaration (an empty interface) such as “Serializable” or “Cloneable”.
a Functional Interface is an interface with just one abstract method declared in it such as Runnable interface that only has run() method.
| Static class | cannot be instantiated using the “new” keyword
|
| Static items | only access other static items. Share resources between multiple users
|
| Static constructor | in non-static class: runs only once when class instantiated first time
|
| In static class: runs only once when any of its static members accessed for the first time | |
| Static members | are allocated in high frequency heap area of the memory |
An “interface” to iterate through and each and every element of a list (it’s not a class).
| Abstract Class | Interface | |
| Multiple inheritance | Doesn’t support | support |
| Data Member | contains | Doesn’t contain |
| Constructor declaration | contains | Doesn’t contain |
| Definition | May contain definition with no implementation | Just a pattern, an empty shell |
| Methods | Can have both abstract & non-abstract methods | Can only have abstract methods |
| Implementation | Can contain both definition and its implementation
Incomplete class, cannot be instantiated |
Can only have the signature of the functionality without any code |
| Access Modifier | Can have all access modifiers for member declaration of functions, subs and properties. | Not allowed (all methods must be implicitly defined as public).
All the members assumed as implicitly public. |
| Homogeneity | is used for implementations of the same type, behavior and status | Is used for implementations that share only method signatures |
| Declaration | Acts as a base class for all other classes, can declare/ use any variable | Not allowed to declare any variables |
| Core vs Peripheral | Core identity of a class & for objects of the same data type | Used to define Peripheral ability of a class |
| Rigid vs Supple | More supple | More rigid |
| Fields definition | Predefined fields & constants | Fields cannot be defined in interfaces |
| Abstract Class | Concrete Class | |
| Type | Base class | Default class |
| Methods | May contain partially implemented methods | All methods are completely implemented |
| Functions | Some or all declared functions are purely virtual | No purely virtual functions |
| Instantiation | Cannot be instantiated | Can be instantiated |
Image Courtesy: tutorials.jenkov.com, stackoverflow.com
A class, interface or method that declares one or more type variables (type parameters).
Advantages:
public Class Box{ ——–> public Class Box<T>{
private Object obj; private T t;
public void set (Object obj){ public void set(T t){
this.obj=obj; this.t=t;
} }
public Object get(){ public T get(){
return obj; return t;
} }
} }
A Singletone class;
Only one time in JVM per class loader would be instantiated. Same instance would be use for repeated calls.
public class OnlyOne {
private static OnlyOne one = new OnlyOne();
// private constructor, so this class cannot be instantiated from outside and prevents
//subclassing.
private OnlyOne();
//a global point of access
public static OnlyOne getInstance() {
return one;
}
}
To use it:
OnlyOne myOne = OnlyOne.getInstance();
| Class | Package | Sub Classes | World | |
| public | ✓ | ✓ | ✓ | ✓ |
| protected | ✓ | ✓ | ✓ | |
| package | ✓ | ✓ | ||
| private | ✓ |
If we don’t use any modifier, it uses default one. The default modifier is package.
| str1==str2 | str1.equals(str2) | |
| String | Compares Objs location | Compares Objs String contents |
| StringBuffer, StringBuilder | Compares Objs location | If and only if str1==str2 would be True. |
So if we need to compare or sort in StringBuffer / StringBuilder we need to turn them into String first. For example:
if (str1.ToString() = str2.ToString()) {
System.out.println(“True”);
} else {
System.out.println(“False”);
}
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