Arrays.sort() ; ---> object[]
Collections.sort()-->list
s = new TreeSet(s); --->sortedset
we can use like this.
Collection c=new ArrayList();
1) if we r storing some integer objects and some Sting objects into array if we r trying to apply sorting then it wil give class cast exception.
2)string object ni object reference ki pas cheya vachu kani object instance ni string refarenceki pass cheyalem we must cast it explicitly.
3) treeset ki pas chesina elements bydefault sorted order inascending.
4) we can cal a static method in non static method directly without object instance.
5) super class lo and child class lo same methods vuna super class reference ni child class ga cast chesina child reference numchi methods cal cheste adi child class methods ne cal chestumdi if it is not available then it go for super .but where as in the constructor point of view it is reverse.
6)try block lo declare chesina variable catch block scope loki radu.
7)inner class instantiation method in outer class .
Line.Point p = new Line.Point();
8)child class lo superclass constructor values veryfirst line of child class constructor cal cheyali if super class dosant declared default constructor we canot cal default constructor in child class.
9) enum type object ki switch default value suport cheyadu.
10)enums ni function out side declare cheyali.
11)subclass refarence ni parent class ki pass cheyavachu kani parent class refarence ni child class ki pass cheyalem.
12) Global and static variable need not be initialized before use but if u specify final then u must intialize. Default value of global and
static int variable is zero. Default value of boolean variable is false. Remember local
variable must be initialized before use.
a) method argument's add(int x,int y) vunna we can pass x value short then internally it follows autoboxing but static method
lo non static method direct ga vadalaem , But non static method lo veroka method ni cal cheyavachu.
2) primitive int value bydefault 0 for either static variable or non static variable and boolean bydefault false
int k; --> 0
boolean istrue; --> false
static int p; --> 0
3) Global and static variable need not be initialized before use but if u specify final then u must intialize.. Default value of global and static int variable is zero. Default value of boolean variable is false. Remember local
variable must be initialized before use.we canot assign a value to final variable second time.
4) $7 is valid identifier. Identifiers must start with a letter, a currency character ($), or
underscore ( _ ). Identifiers cannot start with a number. You can't use a Java keyword as
an identifier. do is a Java keyword.
5) class Animal (super class ) class Cat (child class) and class Test if we pass cat refarence to animal animal in differenct package and default class class then , Cat class won't compile because its superclass, Animal, has default access and is in a different package. Only public superclass can be accessible for different package.
6)If you create object of subclass with reference of super class like ( A a = new B();) then
subclass method and super class variable will be executed.
7) (6Q) As an enum cannot be instantiated using the new operator, the constructors cannot be
called explicitly. You have to do like Test t = BREAKFAST; dont use like Test t=new BREAKFAST
8)First execute static block, then statement block then constructor.
9)By placing a zero in front of the number is an integer in octal form. 010 is in octal form
so its value is 8.
10) int or smaller expressions always resulting in an int. So compiler complain about Type
mismatch: cannot convert from int to byte for b = b+7; But b += 7; // No problem
because +=, -=, *=, and /= will all put in an implicit cast. b += 7 is same as b = (byte)b+7
so compiler not complain.
byte b=8;
b+=8; // as b = (byte)b+8
b=b+7; //wrong
11) Java pass reference as value. passing the object reference, and not the actual object itself.
Simply reassigning to the parameter used to pass the value into the method will do
nothing, because the parameter is essentially a local variable.
12) Primitive widening uses the smallest method argument possible. (For Example if you
pass short value to a method but method with short argument is not available then
compiler choose method with int argument). But in this case compiler will prefer the
older style before it chooses the newer style, to keep existing code more robust. var-args
method is looser than widen.
13) no of repetetion words program
public class Test {
public static void main(String args[]) {
String text = "hi this is english is as so is ";
//String[] words = new String[] {"aaa", "bbb", "ccc", "aaa","aaa","AAA"};
String[] words = text.split(" ");
Map<String, Integer> m = new TreeMap<String, Integer>();
for (String word : words) {
Integer freq = m.get(word);
System.out.println("-freq-->" + freq + "-word-->" + word);
m.put(word, freq == null ? 1 : freq + 1);
}
System.out.println(m);
}
}
14) javac -d classes src/Test.java
in the above command from src folder we are moving class file to classes
15) superclass method with throws exeception ni subclass lo Overridden cheste adi child class lo must ga exception handle cheyali
ex: public int doIt(String str, Integer... data)throws Exception{
it throws like --> Unhandled exception type Exception.
16) doubt
for(int i = 1; i < 4; i++)
for(int j = 2; j < 4; j++)
if(i < j)
assert i!=j : i;
17) n struts html:form use cheste path ki .do avsaram ledu adenormal form ayite .do vadali.
18) From J2SE 5.0 onwards. return type in the overriding method can be same or subtype of
the declared return type of the overridden (superclass) method.
19) You can not override private method , private method is not availabe in subclass .
and a class have show mwthod and b class also show method but b extends a then if i cla the show method method form b object then it will cla from b class ;
20) The overriding method can throw any unchecked (runtime) exception, regardless of
exception thrown by overridden method. NullPointerException is RuntimeException so
compiler not complain.
parent lo Filenotfound
child lo NulllPointer
ex : A a=new B();
a.show();
21) in interface if we use the method in this fo((B)b).test2(); is proper cast. test2() method is in class B so need to cast b then only
test2() is accessible. (B)b.test2(); is not proper cast without the second set of parentheses,
the compiler thinks it is an incomplete statement.rmat then
protected String getName();
Illegal modifier for the interface method InfA.getName(); only public and abstract are
permitted
22) if there is already a constructor in ur class (public D(int i,int j)), the compiler won't supply a default constructor. If you want a no-argument constructor to overload the witharguments version you already have, you have to define it by yourself. The constructor D() is undefined in class D. If you define explicit constructor then default constructor will not be available. You have to define explicitly like public D(){ } then the above code will
work. If no constructor into your class , a default constructor will be automatically
generated by the compiler.
23) ((B)b).test2(); is proper cast. test2() method is in class B so need to cast b then only
test2() is accessible. (B)b.test2(); is not proper cast without the second set of parentheses,
the compiler thinks it is an incomplete statement.
24) x *= 3 + 7; is same as x = x * (3 +7) = 5 * (10) = 50 because expression on the right is
always placed inside parentheses.
Foo fo=(Alpha)(Beta)(Delta)x;
Foo -->interface
Alpha -->super class
Beta --> child class
Delta --> child class
11)String jstr=json==null?"":json.toString();
12) 34 pending
25) An array is always an instance of Object
26) If the left hand operand is not a String then + operator treat as plus BUT if left hand
operand is a String then + perform String concatenation.
int a =5 , b=6, c =7;
System.out.println("Value is "+ b +c);
System.out.println(a + b +c);
System.out.println("String "+(b+c));
27) On the time of deserialization , the Serializable object not create new object. So
constructor of class B does not called. A is not Serializable object so constructor is called.
28)if Class A does not implements Serializable interface. So throws NotSerializableException
on trying to Serialize a non Serializable object.
29)static variables are not serialaizable
