Questions tagged [java]
Java is a high-level object-oriented programming language. Use this tag when you're having problems using or understanding the language itself. This tag is frequently used alongside other tags for libraries and/or frameworks used by Java developers.
1,908,661
questions
27101
votes
25
answers
1.8m
views
Why is processing a sorted array faster than processing an unsorted array?
In this C++ code, sorting the data (before the timed region) makes the primary loop ~6x faster:
#include <algorithm>
#include <ctime>
#include <iostream>
int main()
{
// ...
- 495k
7733
votes
91
answers
2.6m
views
Is Java "pass-by-reference" or "pass-by-value"?
I always thought Java uses pass-by-reference. However, I read a blog post which claims that Java uses pass-by-value. I don't think I understand the distinction the author is making.
What is the ...
7511
votes
10
answers
792k
views
Why is subtracting these two epoch-milli Times (in year 1927) giving a strange result?
If I run the following program, which parses two date strings referencing times 1 second apart and compares them:
public static void main(String[] args) throws ParseException {
SimpleDateFormat sf ...
- 194k
4678
votes
64
answers
2.6m
views
How do I read / convert an InputStream into a String in Java?
If you have a java.io.InputStream object, how should you process that object and produce a String?
Suppose I have an InputStream that contains text data, and I want to convert it to a String, so for ...
- 47.6k
4381
votes
66
answers
1.4m
views
How do I avoid checking for nulls in Java?
I use x != null to avoid NullPointerException. Is there an alternative?
if (x != null) {
// ...
}
4290
votes
35
answers
1.7m
views
What are the differences between a HashMap and a Hashtable in Java?
What are the differences between a HashMap and a Hashtable in Java?
Which is more efficient for non-threaded applications?
- 51.5k
4076
votes
59
answers
5.1m
views
How do I generate random integers within a specific range in Java?
How do I generate a random int value in a specific range?
The following methods have bugs related to integer overflow:
randomNum = minimum + (int)(Math.random() * maximum);
// Bug: `randomNum` can be ...
- 49k
4075
votes
42
answers
1.8m
views
Create ArrayList from array
Given an array of type Element[]:
Element[] array = {new Element(1), new Element(2), new Element(3)};
How do I convert this array into an object of type ArrayList<Element>?
ArrayList<Element&...
- 53.9k
3997
votes
12
answers
363k
views
Proper use cases for Android UserManager.isUserAGoat()?
I was looking at the new APIs introduced in Android 4.2.
While looking at the UserManager class I came across the following method:
public boolean isUserAGoat()
Used to determine whether the ...
- 71.7k
3940
votes
45
answers
3.3m
views
How do I efficiently iterate over each entry in a Java Map?
If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map?
Will the ordering of ...
- 38.3k
3839
votes
12
answers
316k
views
Why don't Java's +=, -=, *=, /= compound assignment operators require casting long to int?
Until today, I thought that for example:
i += j;
Was just a shortcut for:
i = i + j;
But if we try this:
int i = 5;
long j = 8;
Then i = i + j; will not compile but i += j; will compile fine.
...
- 37.4k
3826
votes
17
answers
507k
views
Why is char[] preferred over String for passwords?
In Swing, the password field has a getPassword() (returns char[]) method instead of the usual getText() (returns String) method. Similarly, I have come across a suggestion not to use String to handle ...
- 39.3k
3751
votes
7
answers
4.4m
views
Iterate through a HashMap [duplicate]
What's the best way to iterate over the items in a HashMap?
- 57.4k
3707
votes
61
answers
744k
views
How can I create a memory leak in Java?
I just had an interview where I was asked to create a memory leak with Java.
Needless to say, I felt pretty dumb, having no idea how to start creating one.
What would an example be?
3656
votes
31
answers
2.5m
views
What is the difference between public, protected, package-private and private in Java?
In Java, are there clear rules on when to use each of access modifiers, namely the default (package private), public, protected and private, while making class and interface and dealing with ...
- 38.1k
3590
votes
33
answers
1.4m
views
When to use LinkedList over ArrayList in Java?
I've always been one to simply use:
List<String> names = new ArrayList<>();
I use the interface as the type name for portability, so that when I ask questions such as this, I can rework ...
- 39.2k
3484
votes
30
answers
6.7m
views
How do I convert a String to an int in Java?
How can I convert a String to an int?
"1234" → 1234
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3400
votes
25
answers
1.1m
views
What is a serialVersionUID and why should I use it?
Eclipse issues warnings when a serialVersionUID is missing.
The serializable class Foo does not declare a static final
serialVersionUID field of type long
What is serialVersionUID and why is ...
- 80.1k
3299
votes
34
answers
3.7m
views
Initialization of an ArrayList in one line
I wanted to create a list of options for testing purposes. At first, I did this:
ArrayList<String> places = new ArrayList<String>();
places.add("Buenos Aires");
places.add("Córdoba");
...
- 91.9k
3193
votes
59
answers
1.2m
views
How do I test a class that has private methods, fields or inner classes?
How do I use JUnit to test a class that has internal private methods, fields or nested classes?
It seems bad to change the access modifier for a method just to be able to run a test.
2979
votes
3
answers
256k
views
Why is printing "B" dramatically slower than printing "#"?
I generated two matrices of 1000 x 1000:
First Matrix: O and #.
Second Matrix: O and B.
Using the following code, the first matrix took 8.52 seconds to complete:
Random r = new Random();
for (int i = ...
- 26.6k
2873
votes
33
answers
1.9m
views
How can I create an executable/runnable JAR with dependencies using Maven?
I want to package my project in a single executable JAR for distribution.
How can I make a Maven project package all dependency JARs into my output JAR?
- 29.3k
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votes
64
answers
1.5m
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How can I fix 'android.os.NetworkOnMainThreadException'?
I got an error while running my Android project for RssReader.
Code:
URL url = new URL(urlToRssFeed);
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser parser = factory....
- 29.4k
2687
votes
31
answers
2.6m
views
How do I determine whether an array contains a particular value in Java?
I have a String[] with values like so:
public static final String[] VALUES = new String[] {"AB","BC","CD","AE"};
Given String s, is there a good way of testing whether VALUES contains s?
- 33.7k
2677
votes
52
answers
603k
views
Does a finally block always get executed in Java?
Considering this code, can I be absolutely sure that the finally block always executes, no matter what something() is?
try {
something();
return success;
}
catch (Exception e) {
...
- 27.2k
2652
votes
22
answers
1.1m
views
How do I call one constructor from another in Java?
Is it possible to call a constructor from another (within the same class, not from a subclass)? If yes how? And what could be the best way to call another constructor (if there are several ways to do ...
- 80.1k
2540
votes
29
answers
1.1m
views
What's the difference between @Component, @Repository & @Service annotations in Spring?
Can @Component, @Repository and @Service annotations be used interchangeably in Spring or do they provide any particular functionality besides acting as a notation device?
In other words, if I have a ...
- 25.5k
2528
votes
24
answers
1.0m
views
What is reflection and why is it useful?
What is reflection, and why is it useful?
I'm particularly interested in Java, but I assume the principles are the same in any language.
- 47.7k
2513
votes
31
answers
5.6m
views
How do I declare and initialize an array in Java?
How do I declare and initialize an array in Java?
- 26.4k
2388
votes
37
answers
3.1m
views
What's the simplest way to print a Java array?
In Java, arrays don't override toString(), so if you try to print one directly, you get the className + '@' + the hex of the hashCode of the array, as defined by Object.toString():
int[] intArray = ...
- 54.2k
2354
votes
32
answers
1.6m
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How to get an enum value from a string value in Java
Say I have an enum which is just
public enum Blah {
A, B, C, D
}
and I would like to find the enum value of a string, for example "A" which would be Blah.A. How would it be possible to ...
- 33.2k
2350
votes
42
answers
760k
views
"implements Runnable" vs "extends Thread" in Java
From what time I've spent with threads in Java, I've found these two ways to write threads:
With implements Runnable:
public class MyRunnable implements Runnable {
public void run() {
//...
- 24.2k
2334
votes
23
answers
780k
views
What is a JavaBean exactly?
I understood, I think, that a "Bean" is a Java-class with properties and getters/setters.
As much as I understand, it is the equivalent of a C struct. Is that true?
Also, is there a real ...
- 77k
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votes
35
answers
1.8m
views
How do you assert that a certain exception is thrown in JUnit tests?
How can I use JUnit idiomatically to test that some code throws an exception?
While I can certainly do something like this:
@Test
public void testFooThrowsIndexOutOfBoundsException() {
boolean ...
- 57.4k
2151
votes
15
answers
1.0m
views
Comparing Java enum members: == or equals()?
I know that Java enums are compiled to classes with private constructors and a bunch of public static members. When comparing two members of a given enum, I've always used .equals(), e.g.
public ...
- 355k
2125
votes
12
answers
1.2m
views
How to use java.net.URLConnection to fire and handle HTTP requests
Use of java.net.URLConnection is asked about pretty often here, and the Oracle tutorial is too concise about it.
That tutorial basically only shows how to fire a GET request and read the response. It ...
2104
votes
28
answers
1.9m
views
Does Java support default parameter values?
I came across some Java code that had the following structure:
public MyParameterizedFunction(String param1, int param2)
{
this(param1, param2, false);
}
public MyParameterizedFunction(String ...
- 25.2k
2065
votes
28
answers
923k
views
Java inner class and static nested class
What is the main difference between an inner class and a static nested class in Java? Does design / implementation play a role in choosing one of these?
- 27.7k
2060
votes
37
answers
1.4m
views
How do I break out of nested loops in Java?
I've got a nested loop construct like this:
for (Type type : types) {
for (Type t : types2) {
if (some condition) {
// Do something and break...
break; // Breaks ...
- 24.2k
1960
votes
46
answers
1.6m
views
How to generate a random alpha-numeric string
I've been looking for a simple Java algorithm to generate a pseudo-random alpha-numeric string. In my situation it would be used as a unique session/key identifier that would "likely" be unique over ...
1912
votes
38
answers
4.8m
views
How do I split a string in Java?
I want to split the string "004-034556" into two strings by the delimiter "-":
part1 = "004";
part2 = "034556";
That means the first string will contain the ...
- 21.4k
1891
votes
15
answers
221k
views
Why does this code using random strings print "hello world"?
The following print statement would print "hello world".
Could anyone explain this?
System.out.println(randomString(-229985452) + " " + randomString(-147909649));
And randomString() looks like this:
...
- 20.9k
1873
votes
65
answers
1.7m
views
Sort a Map<Key, Value> by values
I need to sort a Map<Key, Value> on the values.
Since the values are not unique, I find myself converting the keySet into an array, and sorting that array through array sort with a custom ...
1867
votes
64
answers
4.3m
views
What does "Could not find or load main class" mean?
A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class ...
What does this mean, what causes it, and how ...
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votes
37
answers
540k
views
Why use getters and setters/accessors?
What's the advantage of using getters and setters - that only get and set - instead of simply using public fields for those variables?
If getters and setters are ever doing more than just the simple ...
- 39.4k
1763
votes
31
answers
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views
How can I avoid Java code in JSP files, using JSP 2?
I know that something like the following three lines
<%= x+1 %>
<%= request.getParameter("name") %>
<%! counter++; %>
is an old school way of coding and in JSP version 2 ...
- 18.6k
1755
votes
35
answers
1.7m
views
How do I create a Java string from the contents of a file?
I've been using the idiom below for some time now. And it seems to be the most wide-spread, at least on the sites I've visited.
Is there a better/different way to read a file into a string in Java?
...
- 196k
1732
votes
33
answers
881k
views
Difference between StringBuilder and StringBuffer
What is the main difference between StringBuffer and StringBuilder?
Is there any performance issues when deciding on any one of these?
- 19k
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votes
51
answers
2.2m
views
How to fix java.lang.UnsupportedClassVersionError: Unsupported major.minor version
I am trying to use Notepad++ as my all-in-one tool edit, run, compile, etc.
I have JRE installed, and I have setup my path variable to the .../bin directory.
When I run my "Hello world" in Notepad++,...
- 23.8k
1715
votes
29
answers
2.9m
views
In detail, how does the 'for each' loop work in Java?
Consider:
List<String> someList = new ArrayList<String>();
// add "monkey", "donkey", "skeleton key" to someList
for (String item : someList) {
System....
- 31.9k