Develop Reference

Java Method Example clarification

I apologize if this question is uber-simplistic, but I'm still in the early stages of learning Java. I have an example program that calls other methods within the class, and I'm not totally following a few of the elements - hoping someone can clarify. It's a simply random number guessing game and it works fine, but I want to better understand some components. Specifically:
There is a boolean variable (validInput) that is declared but never appears to be used anywhere in the methods
There are 2 methods (askForAnotherRound and getGuess) with a 'while' loop that just has 'true' as the variable(?) - i.e. "while (true)."
This code is directly from the example in the book and, again, it works. I just want to better understand those 2 elements above. (I think the validInput variable is not useful as when I 'comment out' that line the code still executes). I'm curious, though, about the "while (true)" element. There is an option to set, in the askForAnotherRound, to set the return value to false (ending the program). Are boolean methods defaulted to 'true' when they are first executed/called?
Again...understand this is probably a super-simple question for most folks on here, but as a newb I just want to understand this as best I can...
Thanks!!!
// Replicates the number guessing game using 4 separate methods.
import java.util.Scanner;
public class GuessingGameMethod2
{
static Scanner sc = new Scanner(System.in);
public static void main(String[] args)
{
System.out.println("Let's play a guessing game!");
do
{
playARound();
}while (askForAnotherRound());
System.out.println("Thanks for playing!");
}
public static void playARound()
{
boolean validInput;
int number, guess;
String answer;
//Pick a Random Number
number = getRandomNumber();
//Get a guess
System.out.println("\nI'm thinking of a number between 1 and 10.");
System.out.print("What do you think it is? ");
guess = getGuess();
//Check the guess
if (guess == number)
System.out.println("You're right!");
else
System.out.println("You're wrong! The number was " + number + ".");
}
public static int getRandomNumber()
{
return (int)(Math.random() * 10) + 1;
}
public static int getGuess()
{
while (true)
{
int guess = sc.nextInt();
if (guess < 1 || guess > 10)
{
System.out.print("I said, between 1 and 10. Try again");
}
else
return guess;
}
}
public static boolean askForAnotherRound()
{
while (true)
{
String answer;
System.out.print("\nPlay again? Y or N: ");
answer = sc.next();
if (answer.equalsIgnoreCase("Y"))
return true;
else if (answer.equalsIgnoreCase("N"))
return false;
}
}
}

I don't see boolean validInput being used either. But if it were to be used somewhere it would probably be to check that you guess satisfies 1 <= guess <= 10.
When it comes to askForAnotherRound and getGuess here's what you should know:
while(true) is always executed. One way you can get out of the while loop is if you use the break statement or if the loop is in a function you can return something. The method askForAnotherRound() will always return either true or false. Depending on the returned value of askForAnotherRound() you will either play another round or not. Note that when you have
`do{
...
someActions()
...
}while(booleanValue)`
someActions() will be executed at least once before it checks for the value of booleanValue which, if it turns out false you'll exit out of the do/while loop. Boolean methods do not default to anything, you have to give them a value.
Hope this helps! I'm also in the process of learning Java right now, so good luck!

As I see you're absolutely true about validInput - it isn't used. May be it will be used in the following chapters.
As for askForAnotherRound() - no, boolean methods don't evalute to true, by default. Even more, Java compiler throw an error if it find a method which does not return value and finish it execution in some cases.
while(true) - it's infinite loop, so it will be executed untill some instruction which interrupts loop, in general it's return statement.
askForAnotherRound() do the following:
- asks user if he/she want to play again
- returns true if user input "Y"
- returns false if user input "Y"
- asks again in all other cases(so it doesn't finish execution) and etc.
Hope it'll help

The validInput is indeed worthless.
The infinite loops are required to read from the console to get a valid input. e.g
while (true)
//start infinite loop
{
int guess = sc.nextInt();
if (guess < 1 || guess > 10)
{
//continue the loop the input is not between 1-10
System.out.print("I said, between 1 and 10. Try again");
}
else
//break out of infinite loop, valid int
return guess;
}
If we take this method without the infinite loop, and i recommend trying this, it will simply return the value read even if it was not valid.
For example.
return sc.nextInt();
will allow any int returned, if we returned anything outside of the bounds in the current impl it would loop again until you enter a value between 1-10
The same is also true for ask for next round, its looping until a valid input is given.
I would bet the next exercises will use the validInput var as both these methods loop until a valid input is given.

You are right about validInput. It is not used. And probably missed after some code change. Should be removed.
while(true) - true is not variable but a boolean constant. It will basically make program run for ever in this case unless somebody kills program. Another alternative would have been to use break to exit out of loop on some condition.

if-statement with continue or break

I'm doing the simulation of Java OCA test. During my study with the book "OCA - Study Guide" (Sybex), at the second chapter there is the following (Table 2.5, page 91):
if - allows break statement: NO
if - allows continue statement: NO
But during the simulation of Java OCA test, did by Sybex (online book OCA), there is this question:
int x= 5;
while (x>=0) {
int y = 3;
while (y>0) {
if (x<2)
continue;
x--; y--;
System.out.println(x*y + " ");
}
}
My answer was: It doesn't compile
But the correct answer is:
8
3
0
2
What is the correct information?
What is right (book or simulation test)?
This is the table:
Thanks a lot!

I think what you need to understand is that in your code, the if statement is INSIDE a while loop (which is inside another while loop) and that's why it's OK to use continue in that case. An if statement outside of a loop isn't allowed to use continue. But inside of a for loop, while loop or do while loop, you CAN use continue inside an if statement.
So the code that you provided works and the output is:
8
3
0
2
But then I took your code and commented out the two while loops and now the code doesn't compile:
int x= 5;
// while (x>=0) {
int y = 3;
// while (y>0) {
if (x<2)
continue; //<-- COMPILE ERROR: continue cannot be used outside of a loop
x--; y--;
System.out.println(x*y + " ");
// }
// }
The output is:
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
continue cannot be used outside of a loop
I wrote a very detailed Java looping tutorial that covers the continue statement, the break statement, and all the different types of loops in Java.

i think continue here is following the outer while loop not immediate if , since continue is allowed inside while loop it works.
public static void main(String[] args) {
int x= 5;
//while (x>=0) {
int y = 3;
//while (y>0) {
if (x<2)
continue;
x--; y--;
System.out.println(x*y + " ");
}
}
// }
// }
Error:(10, 21) java: continue outside of loop

Both book and test simulators are right. break and continue keywords are not allowed inside if. The continue you see inside if is perfectly valid because it appears within the context of outer while so that's fine. To wrap it all, when you start a while or for block, using break and continue everywhere inside this block is correct, even if it happens to be inside an if block.

normally break and continue are allowed in if statements but you should not use them that often because it is bad Coding stlye.
*unit = unit test framework
A Reason would be for example: If you try to test your software with unit (if you dont know unit not yet dont worry :D) it is much harder to understand what the code does than normally and also to test the code.
Good Luck and have fun hope i can help you

Why iteration of a block is prevented in java?

retry: {
.........
if(xyz < 5) {
continue retry;
}
}
Problem : continue cannot be used outside of a loop
Why iteration of a block is prevented in java?

It's not supported because noone implemented it, presumably because they thought that, well, loops should be implemented with the loop constructs Java provides.
What you want to do can easily be implemented as a while loop if you invert the condition and use break instead:
while (true) {
.........
if(xyz >= 5) {
break;
}
}
Or why not write it as a regular loop without break or continue?
do {
.........
} while (xyz < 5);

Problem : continue cannot be used outside of a loop
continue is used within a loop i.e. for/while/do-while, if statements are conditional code blocks and not loops.
Learn more about code branching statements here:
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html

The purpose of continue keyword is to skip rest of the code inside loop to start with the next pass of loop. So, by it's purpose, continue would not fit in the contexts outside the loops.
goto could be other option for you to reset (or restart) the code of execution. However the best way could be move such part into a method and use recursion along with return under specific conditions to achieve the purpose you are talking about.

As others have pointed out, labels in Java label loops, and are used to specify which loop to break or continue.
That said, if you really want to write "spaghetti code" you can misuse break and continue like for instance:
public class SillyGotoExmple {
public static void main(String args[]) {
int count, loops;
final int NUM_LOOPS=2;
final int MAX_COUNT=10;
loops=0;
bar: do {
count = 0;
foo: do {
System.out.println(count);
if(++count < MAX_COUNT) {
continue foo;
} else if(++loops < NUM_LOOPS) {
continue bar;
} else {
break bar;
}
} while (true);
} while(true);
}
}
For some more discussion on goto in Java, see e.g. Is there a goto statement in Java?

When would a do-while loop be the better than a while-loop?

This is a highly subjective question, so I'll be more specific. Is there any time that a do-while loop would be a better style of coding than a normal while-loop?
e.g.
int count = 0;
do {
System.out.println("Welcome to Java");
count++;
} while (count < 10);`
It doesn't seem to make sense to me to check the while condition after evaluating the do-statement (aka forcing the do statement to run at least once).
For something simple like my above example, I would imagine that:
int count = 0;
while(count < 10) {
System.out.println("Welcome to Java"); count++;
}
would be generally considered to have been written in a better writing style.
Can anyone provide me a working example of when a do-while loop would be considered the only/best option? Do you have a do-while loop in your code? What role does it play and why did you opt for the do-while loop?
(I've got an inkling feeling that the do-while loop may be of use in coding games. Correct me, game developers, if I am wrong!)

If you want to read data from a network socket until a character sequence is found, you first need to read the data and then check the data for the escape sequence.
do
{
// read data
} while ( /* data is not escape sequence */ );

The while statement continually executes a block of statements while a particular condition is true
while (expression) {
statement(s)
}
do-while evaluates its expression at the bottom of the loop, and therefore, the statements within the do block are always executed at least once.
do {
statement(s)
} while (expression);
Now will talk about functional difference,
while-loops consist of a conditional branch instructions such as if_icmpge or if_icmplt and a goto statement. The conditional instruction branches the execution to the instruction immediately after the loop and therefore terminates the loop if the condition is not met. The final instruction in the loop is a goto that branches the byte code back to the beginning of the loop ensuring the byte code keeps looping until the conditional branch is met.
A Do-while-loops are also very similar to for-loops and while-loops except that they do not require the goto instruction as the conditional branch is the last instruction and is be used to loop back to the beginning
A do-while loop always runs the loop body at least once - it skips the initial condition check. Since it skips first check, one branch will be less and one less condition to be evaluated.
By using do-while you may gain performance if the expression/condition is complex, since it is ensured to loop atleast once. In that casedo-while could call for performance gain
Very Impressive findings here,
http://blog.jamesdbloom.com/JavaCodeToByteCode_PartOne.html#while_loop

The do-while loop is basically an inverted version of the while-loop.
It executes the loop statements unconditionally the first time.
It then evaluates the conditional expression specified before executing the statements again.
int sum = 0;
int i = 0;
do
{
sum += ids[i];
i++;
} while (i < 4);
Reference material

Simply, when you want to check condition before and then perform operation while is better option, and if you want to perform operation at least once and then check the condition do-while is better.
As per your question a working example,
1. when I needed to find the field which could be declared in the same class or the super class or the super class of that super class and so on i.e. finding the field located in deep class hierarchy. (A extends B B extends C and so on)
public Field SearchFieldInHierarchy(Object classObj, String fieldName )
{
Field type = null;
Class clz = classObj.getClass();
do
{
try
{
type = clz.getDeclaredField(fieldName);
break;
} catch (NoSuchFieldException e)
{
clz = clz.getSuperclass();
}
} while(clz != null || clz != Object.class);
return type;
}
2. When reading input stream from Http response
do
{
bytesRead = inputStream.read(buffer, totalBytesRead, buffer.length - totalBytesRead);
totalBytesRead += bytesRead;
} while (totalBytesRead < buffer.length && bytesRead != 0);

You kind of answer the question yourself-when it needs to run at least once, and it makes sense to read it that way.

do - while loop allows you to ensure that the piece of code is executed at least once before it goes into the iteration.

In a while loop, the condition is tested before it executes code in the loop. In a do while loop, the code is executed before the condition is tested, resulting in the code always being executed at least once. Example:
$value = 5;
while($value > 10){
echo "Value is greater than 10";
}
The above would never output anything. If we do the same again like this:
$value = 5;
do{
echo "Value is greater than 10";
}while($value > 10)
It would output Value is greater than 10 because the condition is tested after the loop is executed. After this it would not output anything further.

The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once.
For example do check this link: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html

If the looping condition can only be known after a first step of the loop (when you do not want a condition before you enter the loop).
Typically:
do {
expr = ...;
while (expr);

Use the while Statement when you have to check a condition repeatedly and only when the condition is satisfied execute the loop
while(condition) //eg. a>5
{
Body of Loop
}
If you see the flow of control here you can see that the condition is checked before the execution of the loop, if the condition is not met the loop will not execute at all
In the Do-While statement the program will execute the body of the loop once and then it will check if the statement is true or not
do
{
Body of Loop
}
while(condition); //eg. a>5
If you notice the flow of control here you will see that the body is executed once, then the condition is checked. If the condition is False the Program will break out of the loop, if True it will continue executing till the condition is not satisfied
It is to be noted that while and do-while give the same output only the flow of control is different

/*
while loop
5 bucks
1 chocolate = 1 bucks
while my money is greater than 1 bucks
select chocolate
pay 1 bucks to the shopkeeper
money = money - 1
end
come to home and cant go to while shop because my money = 0 bucks
*/
#include<stdio.h>
int main(){
int money = 5;
while( money >= 1){
printf("inside the shopk and selecting chocolate\n");
printf("after selecting chocolate paying 1 bucks\n");
money = money - 1 ;
printf("my remaining moeny = %d\n", money);
printf("\n\n");
}
printf("dont have money cant go inside the shop, money = %d", money);
return 0;
}
infinite money
while( codition ){ // condition will always true ....infinite loop
statement(s)
}
please visit this video for better understanding
https://www.youtube.com/watch?v=eqDv2wxDMJ8&t=25s

It is very simple to distinguish between the two. Let's take While loop first.
The syntax of while loop is as follows:
// expression value is available, and its value "matter".
// if true, while block will never be executed.
while(expression) {
// When inside while block, statements are executed, and
// expression is again evaluated to check the condition.
// If the condition is true, the while block is again iterated
// else it exists the while block.
}
Now, let's take the do-while loop.
The syntax of do-while is different:
// expression value is available but "doesn't matter" before this loop, & the
// control starts executing the while block.
do {
// statements are executed, and the
// statements is evaluated and to check the condition. If true
// the while block is iterated, else it exits.
} while(expression);
A sample program is given below to make this concept clear:
public class WhileAndDoWhile {
public static void main(String args[]) {
int i = 10;
System.out.println("While");
while (i >= 1) {
System.out.println(i);
i--;
}
// Here i is already 0, not >= 1.
System.out.println("do-while");
do {
System.out.println(i);
i--;
} while (i >= 1);
}
}
Compile and run this program, and the difference becomes apparent.

What is the "continue" keyword and how does it work in Java?

I saw this keyword for the first time and I was wondering if someone could explain to me what it does.
What is the continue keyword?
How does it work?
When is it used?

continue is kind of like goto. Are you familiar with break? It's easier to think about them in contrast:
break terminates the loop (jumps to the code below it).
continue terminates the rest of the processing of the code within the loop for the current iteration, but continues the loop.

A continue statement without a label will re-execute from the condition the innermost while or do loop, and from the update expression of the innermost for loop. It is often used to early-terminate a loop's processing and thereby avoid deeply-nested if statements. In the following example continue will get the next line, without processing the following statement in the loop.
while (getNext(line)) {
if (line.isEmpty() || line.isComment())
continue;
// More code here
}
With a label, continue will re-execute from the loop with the corresponding label, rather than the innermost loop. This can be used to escape deeply-nested loops, or simply for clarity.
Sometimes continue is also used as a placeholder in order to make an empty loop body more clear.
for (count = 0; foo.moreData(); count++)
continue;
The same statement without a label also exists in C and C++. The equivalent in Perl is next.
This type of control flow is not recommended, but if you so choose you can also use continue to simulate a limited form of goto. In the following example the continue will re-execute the empty for (;;) loop.
aLoopName: for (;;) {
// ...
while (someCondition)
// ...
if (otherCondition)
continue aLoopName;

Let's see an example:
int sum = 0;
for(int i = 1; i <= 100 ; i++){
if(i % 2 == 0)
continue;
sum += i;
}
This would get the sum of only odd numbers from 1 to 100.

If you think of the body of a loop as a subroutine, continue is sort of like return. The same keyword exists in C, and serves the same purpose. Here's a contrived example:
for(int i=0; i < 10; ++i) {
if (i % 2 == 0) {
continue;
}
System.out.println(i);
}
This will print out only the odd numbers.

Generally, I see continue (and break) as a warning that the code might use some refactoring, especially if the while or for loop declaration isn't immediately in sight. The same is true for return in the middle of a method, but for a slightly different reason.
As others have already said, continue moves along to the next iteration of the loop, while break moves out of the enclosing loop.
These can be maintenance timebombs because there is no immediate link between the continue/break and the loop it is continuing/breaking other than context; add an inner loop or move the "guts" of the loop into a separate method and you have a hidden effect of the continue/break failing.
IMHO, it's best to use them as a measure of last resort, and then to make sure their use is grouped together tightly at the start or end of the loop so that the next developer can see the "bounds" of the loop in one screen.
continue, break, and return (other than the One True Return at the end of your method) all fall into the general category of "hidden GOTOs". They place loop and function control in unexpected places, which then eventually causes bugs.

"continue" in Java means go to end of the current loop,
means: if the compiler sees continue in a loop it will go to the next iteration
Example: This is a code to print the odd numbers from 1 to 10
the compiler will ignore the print code whenever it sees continue moving into the next iteration
for (int i = 0; i < 10; i++) {
if (i%2 == 0) continue;
System.out.println(i+"");
}

As already mentioned continue will skip processing the code below it and until the end of the loop. Then, you are moved to the loop's condition and run the next iteration if this condition still holds (or if there is a flag, to the denoted loop's condition).
It must be highlighted that in the case of do - while you are moved to the condition at the bottom after a continue, not at the beginning of the loop.
This is why a lot of people fail to correctly answer what the following code will generate.
Random r = new Random();
Set<Integer> aSet= new HashSet<Integer>();
int anInt;
do {
anInt = r.nextInt(10);
if (anInt % 2 == 0)
continue;
System.out.println(anInt);
} while (aSet.add(anInt));
System.out.println(aSet);
*If your answer is that aSet will contain odd numbers only 100%... you are wrong!

Continue is a keyword in Java & it is used to skip the current iteration.
Suppose you want to print all odd numbers from 1 to 100
public class Main {
public static void main(String args[]) {
//Program to print all odd numbers from 1 to 100
for(int i=1 ; i<=100 ; i++) {
if(i % 2 == 0) {
continue;
}
System.out.println(i);
}
}
}
continue statement in the above program simply skips the iteration when i is even and prints the value of i when it is odd.
Continue statement simply takes you out of the loop without executing the remaining statements inside the loop and triggers the next iteration.

Consider an If Else condition. A continue statement executes what is there in a condition and gets out of the condition i.e. jumps to next iteration or condition. But a Break leaves the loop.
Consider the following Program. '
public class ContinueBreak {
public static void main(String[] args) {
String[] table={"aa","bb","cc","dd"};
for(String ss:table){
if("bb".equals(ss)){
continue;
}
System.out.println(ss);
if("cc".equals(ss)){
break;
}
}
System.out.println("Out of the loop.");
}
}
It will print: aa cc Out of the loop.
If you use break in place of continue(After if.), it will just print aa and out of the loop.
If the condition "bb" equals ss is satisfied:
For Continue: It goes to next iteration i.e. "cc".equals(ss).
For Break: It comes out of the loop and prints "Out of the loop. "

The continue statement is used in loop control structure when you need to jump to the next iteration of the loop immediately.
It can be used with for loop or while loop.
The Java continue statement is used to continue the loop. It continues the current flow of the program and skips the remaining code at the specified condition.
In case of an inner loop, it continues the inner loop only.
We can use Java continue statement in all types of loops such as for loop, while loop and do-while loop.
for example
class Example{
public static void main(String args[]){
System.out.println("Start");
for(int i=0; i<10; i++){
if(i==5){continue;}
System.out.println("i : "+i);
}
System.out.println("End.");
}
}
output:
Start
i : 0
i : 1
i : 2
i : 3
i : 4
i : 6
i : 7
i : 8
i : 9
End.
[number 5 is skip]

I'm a bit late to the party, but...
It's worth mentioning that continue is useful for empty loops where all of the work is done in the conditional expression controlling the loop. For example:
while ((buffer[i++] = readChar()) >= 0)
continue;
In this case, all of the work of reading a character and appending it to buffer is done in the expression controlling the while loop. The continue statement serves as a visual indicator that the loop does not need a body.
It's a little more obvious than the equivalent:
while (...)
{ }
and definitely better (and safer) coding style than using an empty statement like:
while (...)
;

continue must be inside a loop Otherwise it showsThe error below:
Continue outside the loop