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import java.util.*; public class Elway { public static void main(String r[]) { ArrayList[] ls = new ArrayList[3]; for(int i=0; i<3; i++) { ls[i] = new ArrayList(); ls[i].add("a" + i); } Object o = ls; do3(ls); for(int i=0; i<3; i++) //insert code here } static Object do3(ArrayList[] a) { for(int i=0; …

List<? extends Integer> list = new ArrayList<Integer>(); for (Integer element : list) { System.out.println(element); List<? super Integer> list = new ArrayList<Integer>(); for (Integer element : list) { System.out.println(element); For the first three lines as Case 1 using extends it compiles fine. But for Case 2 of next three lines, it …

StringBuffer sb = new StringBuffer(new String("Hello")); //Case 1 StringBuffer sb1; //Case 2 String s = new String("Hello"); // Case 2 sb1=s; //Case 2 StringBuffer sb3; //Case 3 sb3 = new String("Hello");//Case 3 Case 1 compiles fine and outputs *Hello* but Case 2 and Case 3 gives compiler error saying incompatible …

public class GenericDemo<E extends GenericDemo> { E innerE; public E doStuff(E e, GenericDemo<E> e2) { //insert code here } public E getE() { return innerE; } } The options are: 1. return e 2. return e2.getE() 3. return e2 4. return e.getE() Option (1) and (2) compile fine. Option (3) …

Does it jump to random position? May be some hardware key on your keyboard is pressed? Check.

foo |-test |-xcom |-A.class |-B.java And two files: package xcom; public class A{} package xcom public class B extends A{} Given above directory structure where root dir is foo. Class B extends A. Question is: Which allows B.java to compile? The answer is: Set the current dir to *test* then …

public class IC1 { private int x = 3; static int y=4; class Inner { public void inmet() { System.out.println(x + " " + y);} } public void do() { Inner i = new Inner(); i.inmet(); } public static void main(String r[]) { IC1 ic = new IC1(); ic.do(); } …

class Account { Long accNum, pwd} public class Banker { public static void main(String r[]) { new Banker().go(); // do more stuff } void go() { Account a1 = new Account(); a1.accNum = new Long("1024"); Account a2 = a1; Account a3 = a2; a3.pwd = a1.accNum.longValue(); a2.pwd = 4455L; } …

<? super Animal> means animal or it's superclass only. Then why inside the method it's allowing to add Dog object even though it's a subclass of Animal? This program compiles and runs! import java.util.*; class Cat {} class Animal{Animal() {System.out.println("Animal constructor");}} class Dog extends Animal{Dog() {System.out.println("Dog constructor");}} public class GenericDemo5 …

import java.util.*; public class GenericDemo4 { public static void main(String r[]) { Set s1 = new HashSet(); s1.add(0); s1.add("1"); dostuff(s1); } static void dostuff(Set<Number> s) { do2(s); Iterator i = s.iterator(); while(i.hasNext()) System.out.println(i.next() + " "); Object [] oa = s.toArray(); for(int x = 0; x<oa.length; x++) System.out.println(oa[x] + " …

class Sample1 { Sample1() {System.out.println("Sample1 const"); } void doStuff(){System.out.println("sample1 dostuff"); } } class Sample2 extends Sample1 { Sample2() {System.out.println("Sample2 const"); } } public class Sample3 extends Sample2 { Sample3() {System.out.println("Sample3 const"); } public static void main(String r[]) { new Sample3().go(); } void go() { super.doStuff(); } } The output is: …

public class Thread7 implements Runnable { public static void main(String r[]) { Thread t = new Thread(new Thread7()); t.start(); public void run() { System.out.println("C"); System.out.println("D"); } System.out.println("A"); try { t.join(); System.out.println("B"); } catch(Exception e){} } } Query1: On compiling this program I get "illegal start of expression" error pointing to …

Given: Sour extends Blue extends Pan `Pan p4 = new Sour();` Then set of 6 new statements given and asked to choose which ones will compile. They are: 1. `Pan p5 = p4;` 2. `Pan p6 = (Blue)p4;` 3. `Blue b2 = (Blue)b4;` 4. `Blue b3 = (Sour)p4;` 5. `Sour …

import java.io.*; public class Mac extends Laptop { public static void main(String r[]) { new Mac().crunch(); } //insert code here } class Laptop { void crunch() throws IOException { } } The question is simple. Insert a code that will make the program compile. Answers are: `void crunch()` or `void …

Is it mandatory to handle an exception that is thrown using the `throw` keyword? If so is there a separate rule for Checked and Unchecked Exception, as in who should be mandatorily caught? public void met() { throw new ArithmeticException

int [] ia1 = {1,2,3}; int [][] ia2; Object o = ia1; ia2 = new int [3][3]; ia2[0] = (int[])o; ia2[0][0] = (int[])o; Answer is "Compilation fails in last line" Explanation: Arrays are objects, and that each array dimension is a separate type. So, for instance, ia2 is of type …

I noticed that when equals() is used to compare between two String objects it works fine. But when I compare two StringBuffer objects or two StringBuilder objects or mix of String and Builder and Buffer, equals() doesn't work even though I give same string value to the two objects being …

import java.util.*; class SampleA { } class SampleB extends SampleA{ } class SampleC extends SampleA{ } class VectorDemo { public static void main(String r[]) { Vector<SampleA> v = new Vector<SampleA>(); v.add(new SampleB()); v.add(new SampleC()); SampleC rect = v.get(2); } } The output says "Incompatible types. Found SampleA, required SampleC. SampleC …

Why does first two eg give true when a check done on them using if(x==y) but 3rd and 4th case give false? eg1: `String x = "Hello", y="Hello" ` eg2: `String x = "Hello";` `String y= "Hello";` eg3: `String x="Hello";` `String y = new String("Hello");` eg4: `String x = new …

The o/p is 10. But shouldn't it be 9? Is it got anything to do with Postfix or variable scoping has a role to play?

Compiles fine. But at runtime it gives ClassCastException. My doubt is that why is it so even though st which is of type Sample1 gets casted to Sample2 and generates exception even though Sample2 IS-A Sample1.

First statement: int[] it = new int[][]{{1}}[0]; //Valid Statement Second statement: int[][]it2 = new int[]{0}[0][0]; //Invalid Statement First one no explanation given. Second one is invalid because it attempts to access a two-dimensional object from a single dimension array. My query: 1. First of all can someone please interpret those …

A question that I came across: Q: Can you pass a double to a method when it's argument is a float? A: No. Regardless of the double value, a double-sized variable is too big to fit in a float, so it can't be implicitly cast. Doubt: Shouldn't narrowing happen implicitly?

class Dog { public void bark() { System.out.println("Dog bark"); } } class Hound extends Dog { public void bark() { System.out.println("Hound bark"); } public void sniff() { System.out.println("Hound sniff"); } } public class CastingMethodCall { public static void main(String r[]) { ((Dog)new Hound()).bark(); //((Dog)new Hound()).sniff(); } } On executing above, …

class Mixer { Mixer() {} Mixer(Mixer m) { m1 = m; } Mixer m1; public static void main(String args[]) { Mixer m2 = new Mixer(); Mixer m3 = new Mixer(m2); m3.go(); Mixer m4 = m3.m1; m4.go(); Mixer m5 = m2.m1; m5.go(); } void go() { System.out.println("hi"); } } The answer …

public class Thread8 implements Runnable { Demo d; public static void main(String r[]) { new Thread8().disp(); } void disp() { d = new Demo(); new Thread(new Thread8()).start(); new Thread(new Thread8()).start(); } public void run() { d.fun(Thread.currentThread().getId()); } } class Demo { static long f=0; synchronized void fun(long id) { for(int …

Here when I execute the program, the output I'm getting on all the tries is 342 (naturally first 34 prints together then 2 seconds later 2 prints). My doubts is that is the order of start() [x.start() followed by y.start()] important in order of execution? Shouldn't the output be 234? …

Two doubts: 1. Placing run() inside main() generates "illegal start of expression error"? So why can't run() be placed inside main()? 2.With the above code I am getting the output as A C D B. Why B comes last is because of join(). But what I'm unable to understand is …

I have a question in SCJP text book by Kathy Sierra and Bert Bates in Development chapter. Q. Given the default classpath: /foo And the directory structure: foo | test | xcom |--A.class |--B.java And these two files as mentioned in the code snippet section. Amongst the choice, 1. javac …

General rules is that: 1) A private method cannot be overridden. 2) Subclass method should be a lower access modifier than a overridden superclass method. But the output of the program is "I am Son". Why? Is it because of the rule 2 the output comes? If that's the case …

Hey guys, I'm a starter so I had a doubt related to placement of a static variable. On line 5, for "static int d=5" compiler issues "illegal start of expression". But when I place it outside main() as Instance variable it works and prints "Hello 5". Why this thing? public …

A snippet: package com.deitel.jhtp7.ch14; public class AccountRecord { ..... Doubt: 1. What does the above package statement imply. As in how(path) this AccountRecord.java will be stored? As in what folder hierarchy should I create? 2. In Command prompt I have set my Current Directory as : E:\RDL\Dropbox\Coding\Core Java\File So will …

[code]public class StringBuilderConstructors { public static void main(String args[]) { StringBuilder sb3 = new StringBuilder("hello"); System.out.printf("buffer3 = %s\n", sb3.toString()); //written in textbook System.out.printf(sb3); //i tried System.out.println("buffer3=" + sb3); // i tried }} o/p: buffer3 = hello hello buffer3=hello[/code] doubt: The book says toString() gives the string representation an object. That …

Hello, I have just started studying EJB. There is this term narrowing in JNDI chapter which I'm frankly stuck at. Could anyone please explain in simple term what exactly it does. And also explain in parts this snippet: [I][B]javax.rmi.PortableRemoteObject.narrow(obj, SignOnHome.class); [/B][/I] Thanks in advance.

[CODE]class Mammal { String name = "furry"; String makeNoise() { return "generic noise";} } class Zebra extends Mammal { String name = "stripes"; String makeNoise() { return "bray";} } public class Zoo { public static void main (String[] args) { new Zoo().go();} void go() { Mammal m = new Zebra(); …