I am compiling a Java program using for loop to find out the biggest value of long. However, nothing was printed when I run the program. Why?
Here's my code:
class LongMaxMin {
public static void main(String args[]) {
long i = 0L;
long result = 0L;
for (; ; ) {
result = i++;
if (i<0)
break;
}
System.out.println("The biggest integer:" + result);
}
Mostly because of time.
A long will have a max of about ~9.22 quintillion. You're starting at zero and incrementing up. That means you need to go through 9 quintillion loops before it wraps over and breaks. I just tried to run 2 billion operations in my javascript console and times out for a couple of minutes before I force quit.
If you sit there and let it run long enough, you'll get your output. Alternatively, start i at something close to the max already, like 9,223,372,036,854,700,000, and see if it still gives you the same issues. In Java 8, adding underscore to numeric literals is allowed. Initializing i to something like 9_223_372_036_854_700_000L will give you something in a more timely manner.
The max long is significantly high, at 9.223372e+18. For specifics, 9,223,372,036,854,775,807 is the number in question. This also contributes to that whole "this works, it'll just take WAY too long" theory.
I was curious how long it would take so I wrote a class to do the same thing. Wrote it with a separate thread to update results to the console every 1 second.
"int" results
1,343,211,433 37.4518434691484288634492200 % left
Max Value: 2,147,483,647
Time Taken (seconds): **1.588**
"long" results
1,220,167,357 99.9999999867709190074470400 % left
2,519,937,368 99.9999999726787843108699600 % left
3,881,970,343 99.9999999579115932059510100 % left
5,210,983,861 99.9999999435023997711689800 % left
6,562,562,290 99.9999999288485570811055300 % left
7,853,387,353 99.9999999148534037050721500 % left
9,137,607,100 99.9999999009298653086103000 % left
10,467,975,104 99.9999998865059865071902600 % left
11,813,910,300 99.9999998719133278719112300 % left
13,183,196,499 99.9999998570674971548090400 % left
...it continues on and on...
1,362,032,97 - difference between the 2nd and 3rd values (1 second)
6,771,768,529 seconds - how many seconds it would take to reach long's max value (Long.MAX_VALUE / 2nd3rdDifference)
6,771,768,529 seconds = 214.73 years (per conversion by google search)
So if my calculations are correct...you'd be dead of old age by the time an average computer calculated the max value of long via incrementing and checking if it's overflowed. Your children would be dead to. Your grandchildren, they might be around when it finished...
Code for Max Value Calculation
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.text.NumberFormat;
public class MainLongMaxTest {
// /*
public static final long MAX_VALUE = Long.MAX_VALUE;
public static long value = 0;
public static long previousValue = 0;
// */
/*
public static final int MAX_VALUE = Integer.MAX_VALUE;
public static int value = 0;
public static int previousValue = 0;
*/
public static boolean done;
public static BigDecimal startTime;
public static BigDecimal endTime;
public static void main(String[] args) {
Runnable task = new StatusPrinterRunnable();
new Thread(task).start(); // code waits 1 second before result printing loop
done = false;
startTime = new BigDecimal(System.currentTimeMillis());
while(value >= 0) {
previousValue = value;
value += 1;
}
endTime = new BigDecimal(System.currentTimeMillis());
done = true;
}
}
class StatusPrinterRunnable implements Runnable {
public static final NumberFormat numberFormat = NumberFormat.getNumberInstance();
private static long SLEEP_TIME = 1000;
#Override
public void run() {
try { Thread.sleep(SLEEP_TIME); } catch (InterruptedException e) { throw new RuntimeException(e); }
while(!MainLongMaxTest.done) {
long value = MainLongMaxTest.value;
//long valuesLeft = MAX_VALUE - value;
BigDecimal maxValueBd = new BigDecimal(MainLongMaxTest.MAX_VALUE);
BigDecimal valueBd = new BigDecimal(value);
BigDecimal differenceBd = maxValueBd.subtract(valueBd);
BigDecimal percentLeftBd = differenceBd.divide(maxValueBd, 25, RoundingMode.HALF_DOWN);
percentLeftBd = percentLeftBd.multiply(new BigDecimal(100));
String numberAsString = numberFormat.format(value);
String percentLeftAsString = percentLeftBd.toString();
String message = "" + numberAsString + "\t" + percentLeftAsString + " % left";
System.out.println(message);
try { Thread.sleep(SLEEP_TIME); } catch (InterruptedException e) { throw new RuntimeException(e); }
}
BigDecimal msTaken = MainLongMaxTest.endTime.subtract(MainLongMaxTest.startTime);
BigDecimal secondsTaken = msTaken.divide(new BigDecimal("1000"));
System.out.println();
System.out.println("Max Value: " + numberFormat.format(MainLongMaxTest.previousValue));
System.out.println("Time Taken (seconds): " + secondsTaken);
}
}
I think your logic is correct just it will take a lot of time to reach that value.
the maximum Long value can hold is Long.MAX_value which is 9223372036854775807L
to speed up the logic, I modified the program as below and got the expected result.
public static void main(String args[]) {
long i = 9223372036854775806L;
long result = 0L;
for (; ; ) {
result = i++;
if (i<0) {
System.out.println("result"+result);
System.out.println("i"+i);
break;
}
}
System.out.println("The biggest integer: is" + result);
}
Output:
result9223372036854775807
i-9223372036854775808
The biggest integer: is9223372036854775807
result has the maximum value it can hold after that it changes to its minimum value.
You can get the result in one step if you take advantage of binary algebra by:
result = -1L >>> 1;
Related
I want to try practicing avoiding using static methods/variables when not needed, because I've heard/seen/been told that you want to avoid using them when you can. I decided to make a simple password cracker in Java:
import java.util.Random;
public class PasswordCracker
{
public static void main(String args[])
{
PasswordCracker pwcSimulation = new PasswordCracker();
long totalTimeSpentCracking = 0;
int numSimulations = 100;
for(int i = 0; i < numSimulations; i++)
{
System.out.println(pwcSimulation.PasswordCrackingSimulation());
}
}
long PasswordCrackingSimulation()
{
long startTime = System.currentTimeMillis();
int upperBound = 999999;
Random rand = new Random();
int randomPassword = rand.nextInt(upperBound);
int passwordGuess;
for(int i = 0; i <= upperBound; i++)
{
passwordGuess = i;
if(passwordGuess == randomPassword)
{
System.out.println("password Guessed correctly, the password was: " + randomPassword);
break;
}
/*else
{
System.out.println("Your inputted password is incorrect, please try again.");
}*/
}
long endTime = System.currentTimeMillis();
long timeSpentCracking = (endTime - startTime);
System.out.println("The program took " + timeSpentCracking + "ms OR ~" + ((timeSpentCracking/1000) % 60) + " seconds to complete");
return timeSpentCracking;
}
}
first instantiated a new class (hopefully i did this they way you should?) to avoid having to use a static method for the method PasswordCrackingSimulation. Now i'm having trouble returning a value from the method. The printline in the loop will always print 0, so I know that it isn't taking the returned value in the method. Any help would be lovely :) just trying to learn
No, you're doing everything correctly.
You're returning how long it takes in milliseconds to crack that password.
The answer is less than 1 millisecond. That 0 you see? That's because your method is returning 0. It is doing that because endTime - startTime is zero.
Just write return 1 to test this out yourself - you'll see your print loop print 1 instead.
I'm using a program to run the Collatz Conjecture (http://en.wikipedia.org/wiki/Collatz_conjecture) from mathematics. I've implemented a class that runs the conjecture algorithm (and gives you back the output) and one that creates a fixed thread pool (with my number of processors: 8) and accepts Callables which are calls for the conjecture algorithm.
I created a HashSet<Callable> for all the numbers between 1 (the input type must be a positive integer) and 400,000. This hangs (seemingly) forever, but lower numbers work out just fine, which is strange. Stranger yet, running it appears to take longer to process these calls than it takes a single thread to process the same amount of information; it also bloats the memory significantly.
For instance, on my computer, the program takes less than a second to perform the algorithm (just one iteration) with 400,000 (the final value) and all the lower values take less time to compute (maybe with the exception of primes, which take longer) I'm running Windows 8.1 with 8GB ram, and 8 logical processors at 2.2Ghz.
Code:
private static void initThreads() throws InterruptedException {
//Files.createDirectories(SEQUENCER_FOLDER_PATH);
//Files.createFile(SEQUENCER_FILE_PATH);
ExecutorService service = Executors.newFixedThreadPool(8, new ThreadFactory() {
private BigInteger count = BigInteger.ZERO;
#Override
public Thread newThread(Runnable r) {
count = count.add(BigInteger.ONE);
return new Thread(r, "Collatz Sequencer Thread: " + count);
}
});
int finalNumber = 400_000;
final HashSet<Callable<Void>> tasks = new HashSet<>(finalNumber);
for (long l = 1; l <= finalNumber; l++) {
final BigInteger number = BigInteger.valueOf(l);
tasks.add(() -> {
CollatzSequencer sequencer = new CollatzSequencer(new BigInteger(number.toString()));
synchronized (dataSet) {
dataSet.put(number, sequencer.init());
}
return null;
});
}
service.invokeAll(tasks);
Thread dataThread = new Thread(() -> {
while (true) {
synchronized (dataSet) {
if (dataSet.size() == finalNumber) {
System.err.println("Values: \n");
for (CollatzSequencer.FinalSequencerReport data : dataSet.values()) {
System.err.println("Entry: " + data.getInitialValue() + ", " + data.getIterations());
}
System.exit(0);
}
}
}
}, "Collatz Conjecture Data Set Thread");
dataThread.start();
}
Collatz Conjecture Algorithm:
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package com.collatzsequencer.core;
import java.math.BigInteger;
/**
* A sequencer used for computing the collatz sequence.
*
* #author Sarah Szabo
* #version 1.0
*/
public class CollatzSequencer {
private final BigInteger initialValue;
public CollatzSequencer(BigInteger currentValue) {
if (currentValue == null) {
throw new NullPointerException("Value passed can't be null");
} else if (currentValue.compareTo(new BigInteger("1")) < 0) {
throw new NumberFormatException("The value passed to the constructor must be a natural number.");
}
this.initialValue = currentValue;
}
public FinalSequencerReport init() {
return new FinalSequencerReport(performOperation(new SequencerReport(this.initialValue)), this.initialValue);
}
private SequencerReport performOperation(SequencerReport report) {
if (report.getResult().equals(new BigInteger("1"))) {
return new SequencerReport(report.getResult(), report.getIterations(), report.getSequence().length() > 1
? report.getSequence().substring(0, report.getSequence().length() - 3) : "The sequence starts and ends at 1 <Nothing Done>");
} else if (report.getResult().mod(new BigInteger("2")).equals(new BigInteger("0"))) {
BigInteger value = report.getResult().divide(new BigInteger("2"));
return performOperation(new SequencerReport(value, report.getIterations().add(new BigInteger("1")),
report.getSequence() + " " + report.getResult() + "/2 -> " + value + " ->"));
} else {
BigInteger value = report.getResult().multiply(new BigInteger("3")).add(new BigInteger("1"));
return performOperation(new SequencerReport(value, report.getIterations()
.add(new BigInteger("1")), report.getSequence() + report.getResult() + " * 3 + 1 ->" + value + " ->"));
}
}
public static final class FinalSequencerReport extends SequencerReport {
private final BigInteger initialValue;
private final String finalFormattedString;
public FinalSequencerReport(SequencerReport finalReport, BigInteger initialValue) {
super(finalReport.getResult(), finalReport.getIterations(), finalReport.getSequence());
this.initialValue = initialValue;
this.finalFormattedString = "Initial Value: "
+ getInitialValue() + "\nFinal Value: " + getResult() + "\nIterations: "
+ getIterations() + "\nAlgebraic Sequence:\n" + getSequence();
}
public String getFinalFormattedString() {
return finalFormattedString;
}
public BigInteger getInitialValue() {
return initialValue;
}
}
public static class SequencerReport {
private final BigInteger result, iterations;
private final String sequence;
public SequencerReport(BigInteger result) {
this(result, new BigInteger("0"), "");
}
public SequencerReport(BigInteger result, BigInteger iterations, String sequence) {
this.result = result;
this.iterations = iterations;
this.sequence = sequence;
}
public BigInteger getResult() {
return this.result;
}
public BigInteger getIterations() {
return this.iterations;
}
public String getSequence() {
return this.sequence;
}
}
}
As you said, your code works; the problem is probably just performance. Some things I would try:
Use long instead of BigInteger. BigInteger is very slow.
Instead of mod 2 (or % 2), use & 1. The binary AND will have effectively the same result and is much faster.
You are doing way, way too much String manipulation. Override sequencerReport.toString() and have it do the toString calls all at the end when you're printing the data.
Don't do new ThreadFactory(). Use Guava's ThreadFactoryBuilder.
You should never call new Thread() ever in your code unless you really know what you're doing, which means don't do it.
Add a wait/notify mechanism for dataThread instead of a busy loop. Call dataSet.notify() when the work is done and dataSet.wait() inside the dataThread body.
Given the following pieces of code, which one is more efficient? The real method returnSomething() can also return 0 in reality so try/catch is needed.
//piece one
long sleepTime = 200;
try{ sleepTime /= returnSomething();}
catch(Exception e){sleepTime = 200;}
private int returnSomething(){
return 1;
}
//or
//piece two
long sleepTime = 200;
if(returnSomething() == 3){sleepTime = 67;}
else if(returnSomething() == 2){sleepTime = 100;}
else if(returnSomething() == 1){sleepTime = 200;}
private int returnSomething(){
return 1;
}
I was trying to figure out which piece of code is more efficient in terms of processor usage, they bot do the same thing. I would like to know if the code I wrote for testing is fit for the purpose or whether I can do other kind of testing on the code. My findings show that piece 2 is 9 times more efficient (9 times less time to execute) even though it uses hard-code if statements and the last if statement is always executed.
Full working program
public class CodePerformanceTester
{
public static void main(String[] args){
CodePerformanceTester tester = new CodePerformanceTester();
tester.start();
}
public void start(){
double start = System.currentTimeMillis();
long sleepTime = 200;
for(int i=0; i<10000000; i++){
//uncoment here the two lines below
//try{ sleepTime /= returnSomething();}
//catch(Exception e){sleepTime = 200;}
//coment the IF STATEMENTS when above code uncomented
if(returnSomething() == 3){sleepTime = 67;}
else if(returnSomething() == 2){sleepTime = 100;}
else if(returnSomething() == 1){sleepTime = 200;}
}
double end = System.currentTimeMillis();
System.out.println("Execution time for 10 million iteration was "+(end-start)+" ms.");
}
private int returnSomething(){
return 1;
}
}
The first piece is probably slower because dividing is more expensive than comparing values.
I need a byte generator that would generate values from Byte.MIN_VALUE to Byte.MAX_VALUE. When it reaches MAX_VALUE, it should start over again from MIN_VALUE.
I have written the code using AtomicInteger (see below); however, the code does not seem to behave properly if accessed concurrently and if made artificially slow with Thread.sleep() (if no sleeping, it runs fine; however, I suspect it is just too fast for concurrency problems to show up).
The code (with some added debug code):
public class ByteGenerator {
private static final int INITIAL_VALUE = Byte.MIN_VALUE-1;
private AtomicInteger counter = new AtomicInteger(INITIAL_VALUE);
private AtomicInteger resetCounter = new AtomicInteger(0);
private boolean isSlow = false;
private long startTime;
public byte nextValue() {
int next = counter.incrementAndGet();
//if (isSlow) slowDown(5);
if (next > Byte.MAX_VALUE) {
synchronized(counter) {
int i = counter.get();
//if value is still larger than max byte value, we reset it
if (i > Byte.MAX_VALUE) {
counter.set(INITIAL_VALUE);
resetCounter.incrementAndGet();
if (isSlow) slowDownAndLog(10, "resetting");
} else {
if (isSlow) slowDownAndLog(1, "missed");
}
next = counter.incrementAndGet();
}
}
return (byte) next;
}
private void slowDown(long millis) {
try {
Thread.sleep(millis);
} catch (InterruptedException e) {
}
}
private void slowDownAndLog(long millis, String msg) {
slowDown(millis);
System.out.println(resetCounter + " "
+ (System.currentTimeMillis()-startTime) + " "
+ Thread.currentThread().getName() + ": " + msg);
}
public void setSlow(boolean isSlow) {
this.isSlow = isSlow;
}
public void setStartTime(long startTime) {
this.startTime = startTime;
}
}
And, the test:
public class ByteGeneratorTest {
#Test
public void testGenerate() throws Exception {
ByteGenerator g = new ByteGenerator();
for (int n = 0; n < 10; n++) {
for (int i = Byte.MIN_VALUE; i <= Byte.MAX_VALUE; i++) {
assertEquals(i, g.nextValue());
}
}
}
#Test
public void testGenerateMultiThreaded() throws Exception {
final ByteGenerator g = new ByteGenerator();
g.setSlow(true);
final AtomicInteger[] counters = new AtomicInteger[Byte.MAX_VALUE-Byte.MIN_VALUE+1];
for (int i = 0; i < counters.length; i++) {
counters[i] = new AtomicInteger(0);
}
Thread[] threads = new Thread[100];
final CountDownLatch latch = new CountDownLatch(threads.length);
for (int i = 0; i < threads.length; i++) {
threads[i] = new Thread(new Runnable() {
public void run() {
try {
for (int i = Byte.MIN_VALUE; i <= Byte.MAX_VALUE; i++) {
byte value = g.nextValue();
counters[value-Byte.MIN_VALUE].incrementAndGet();
}
} finally {
latch.countDown();
}
}
}, "generator-client-" + i);
threads[i].setDaemon(true);
}
g.setStartTime(System.currentTimeMillis());
for (int i = 0; i < threads.length; i++) {
threads[i].start();
}
latch.await();
for (int i = 0; i < counters.length; i++) {
System.out.println("value #" + (i+Byte.MIN_VALUE) + ": " + counters[i].get());
}
//print out the number of hits for each value
for (int i = 0; i < counters.length; i++) {
assertEquals("value #" + (i+Byte.MIN_VALUE), threads.length, counters[i].get());
}
}
}
The result on my 2-core machine is that value #-128 gets 146 hits (all of them should get 100 hits equally as we have 100 threads).
If anyone has any ideas, what's wrong with this code, I'm all ears/eyes.
UPDATE: for those who are in a hurry and do not want to scroll down, the correct (and shortest and most elegant) way to solve this in Java would be like this:
public byte nextValue() {
return (byte) counter.incrementAndGet();
}
Thanks, Heinz!
Initially, Java stored all fields as 4 or 8 byte values, even short and byte. Operations on the fields would simply do bit masking to shrink the bytes. Thus we could very easily do this:
public byte nextValue() {
return (byte) counter.incrementAndGet();
}
Fun little puzzle, thanks Neeme :-)
You make the decision to incrementAndGet() based on a old value of counter.get(). The value of the counter can reach MAX_VALUE again before you do the incrementAndGet() operation on the counter.
if (next > Byte.MAX_VALUE) {
synchronized(counter) {
int i = counter.get(); //here You make sure the the counter is not over the MAX_VALUE
if (i > Byte.MAX_VALUE) {
counter.set(INITIAL_VALUE);
resetCounter.incrementAndGet();
if (isSlow) slowDownAndLog(10, "resetting");
} else {
if (isSlow) slowDownAndLog(1, "missed"); //the counter can reach MAX_VALUE again if you wait here long enough
}
next = counter.incrementAndGet(); //here you increment on return the counter that can reach >MAX_VALUE in the meantime
}
}
To make it work one has to make sure the no decisions are made on stale info. Either reset the counter or return the old value.
public byte nextValue() {
int next = counter.incrementAndGet();
if (next > Byte.MAX_VALUE) {
synchronized(counter) {
next = counter.incrementAndGet();
//if value is still larger than max byte value, we reset it
if (next > Byte.MAX_VALUE) {
counter.set(INITIAL_VALUE + 1);
next = INITIAL_VALUE + 1;
resetCounter.incrementAndGet();
if (isSlow) slowDownAndLog(10, "resetting");
} else {
if (isSlow) slowDownAndLog(1, "missed");
}
}
}
return (byte) next;
}
Your synchronized block contains only the if body. It should wrap whole method including if statement itself. Or just make your method nextValue synchronized. BTW in this case you do not need Atomic variables at all.
I hope this will work for you. Try to use Atomic variables only if your really need highest performance code, i.e. synchronized statement bothers you. IMHO in most cases it does not.
If I understand you correctly, you care that the results of nextValue are in the range of Byte.MIN_VALUE and Byte.MAX_VALUE and you don't care about the value stored in the counter.
Then you can map integers on bytes such that you required enumeration behavior is exposed:
private static final int VALUE_RANGE = Byte.MAX_VALUE - Byte.MIN_VALUE + 1;
private final AtomicInteger counter = new AtomicInteger(0);
public byte nextValue() {
return (byte) (counter.incrementAndGet() % VALUE_RANGE + Byte.MIN_VALUE - 1);
}
Beware, this is untested code. But the idea should work.
I coded up the following version of nextValue using compareAndSet which is designed to be used in a non-synchronized block. It passed your unit tests:
Oh, and I introduced new constants for MIN_VALUE and MAX_VALUE but you can ignore those if you prefer.
static final int LOWEST_VALUE = Byte.MIN_VALUE;
static final int HIGHEST_VALUE = Byte.MAX_VALUE;
private AtomicInteger counter = new AtomicInteger(LOWEST_VALUE - 1);
private AtomicInteger resetCounter = new AtomicInteger(0);
public byte nextValue() {
int oldValue;
int newValue;
do {
oldValue = counter.get();
if (oldValue >= HIGHEST_VALUE) {
newValue = LOWEST_VALUE;
resetCounter.incrementAndGet();
if (isSlow) slowDownAndLog(10, "resetting");
} else {
newValue = oldValue + 1;
if (isSlow) slowDownAndLog(1, "missed");
}
} while (!counter.compareAndSet(oldValue, newValue));
return (byte) newValue;
}
compareAndSet() works in conjunction with get() to manage concurrency.
At the start of your critical section, you perform a get() to retrieve the old value. You then perform some function dependent only on the old value to compute a new value. Then you use compareAndSet() to set the new value. If the AtomicInteger is no longer equal to the old value at the time compareAndSet() is executed (because of concurrent activity), it fails and you must start over.
If you have an extreme amount of concurrency and the computation time is long, it is conceivable that the compareAndSet() may fail many times before succeeding and it may be worth gathering statistics on that if concerns you.
I'm not suggesting that this is a better or worse approach than a simple synchronized block as others have suggested, but I personally would probably use a synchronized block for simplicity.
EDIT: I'll answer your actual question "Why doesn't mine work?"
Your code has:
int next = counter.incrementAndGet();
if (next > Byte.MAX_VALUE) {
As these two lines are not protected by a synchronized block, multiple threads can execute them concurrently and all obtain values of next > Byte.MAX_VALUE. All of them will then drop through into the synchronized block and set counter back to INITIAL_VALUE (one after another as they wait for each other).
Over the years, there has been a huge amount written over the pitfalls of trying to get a performance tweak by not synchronizing when it doesn't seem necessary. For example, see Double Checked Locking
Notwithstanding that Heinz Kabutz is the clean answer to the specific question, ye olde Java SE 8 [March 2014] added AtomicIntger.updateAndGet (and friends). This leads to a more general solution if circumstances required:
public class ByteGenerator {
private static final int MIN = Byte.MIN_VALUE;
private static final int MAX = Byte.MAX_VALUE;
private final AtomicInteger counter = new AtomicInteger(MIN);
public byte nextValue() {
return (byte)counter.getAndUpdate(ByteGenerator::update);
}
private static int update(int old) {
return old==MAX ? MIN : old+1;
}
}
I have a DecimalFormat object which I'm using to format all of my double values to a set number of digits (let's say 2) when I'm displaying them. I would like it to normally format to 2 decimal places, but I always want at least one significant digit. For example, if my value is 0.2 then my formatter spits out 0.20 and that's great. However, if my value is 0.000034 my formatter will spit out 0.00 and I would prefer my formatter spit out 0.00003.
The number formatters in Objective-C do this very simply, I can just set a max number of digits I want to show at 2 and the minimum number of significant digits at 1 and it produces my desired output, but how can I do it in Java?
I appreciate any help anyone can offer me.
Kyle
Edit: I'm interested in rounding the values so 0.000037 displays as 0.00004.
It's not efficient, so if you perform this operation often I'd try another solution, but if you only call it occasionally this method will work.
import java.text.DecimalFormat;
public class Rounder {
public static void main(String[] args) {
double value = 0.0000037d;
// size to the maximum number of digits you'd like to show
// used to avoid representing the number using scientific notation
// when converting to string
DecimalFormat maxDigitsFormatter = new DecimalFormat("#.###################");
StringBuilder pattern = new StringBuilder().append("0.00");
if(value < 0.01d){
String s = maxDigitsFormatter.format(value);
int i = s.indexOf(".") + 3;
while(i < s.length()-1){
pattern.append("0");
i++;
}
}
DecimalFormat df = new DecimalFormat(pattern.toString());
System.out.println("value = " + value);
System.out.println("formatted value = " + maxDigitsFormatter.format(value));
System.out.println("pattern = " + pattern);
System.out.println("rounded = " + df.format(value));
}
}
import java.math.BigDecimal;
import java.math.MathContext;
public class Test {
public static void main(String[] args) {
String input = 0.000034+"";
//String input = 0.20+"";
int max = 2;
int min =1;
System.out.println(getRes(input,max,min));
}
private static String getRes(String input,int max,int min) {
double x = Double.parseDouble(((new BigDecimal(input)).unscaledValue().intValue()+"").substring(0,min));
int n = (new BigDecimal(input)).scale();
String res = new BigDecimal(x/Math.pow(10,n)).round(MathContext.DECIMAL64).setScale(n).toString();
if(n<max){
for(int i=0;i<max;i++){
res+="0";
}
}
return res;
}
}