A related issue that I have come across recently (and which led me to this page, before I figured out what was going on) is that the Eclipse environment can report &quot;Incompatible conditional operand types&quot; in an &#39;instanceof&#39; expression erroneously due to a missing &#39;import&#39; statement for the type on the right of the &#39;instanceof&#39;. I spent a while trying to figure out how the types in question could possibly be incompatible before figuring out that a missing import was causing the whole problem. Hopefully this information saves somebody some time.


A more blatant incarnation of your problem is the following:

    if (&quot;foo&quot; instanceof Number)
       // &quot;Incompatible conditional operand types String and Number&quot;

This is specified in [JLS 15.20.2 Type comparison operator `instanceof`](http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2):

&gt;     RelationalExpression:
           RelationalExpression instanceof ReferenceType

&gt; If a cast of the _RelationalExpression_ to the _ReferenceType_ would be rejected as a compile-time error, then the `instanceof` relational expression likewise produces a compile-time error. In such a situation, the result of the `instanceof` expression could never be true.

That is, since this cast expression generates a compile time error:

    (Number) &quot;foo&quot;

so must this expression:

    (&quot;foo&quot; instanceof Number)

---

Your case is a bit more subtle, but the principle is the same:

* `String` is a final class
* `String` does not implement `Cloneable`
* Therefore you can&#39;t do `(Cloneable) aString`
* Therefore also you can&#39;t do `aString instanceof Cloneable`

The compiler knows that `String` is a final class and doesn&#39;t implement `Cloneable`. So no instance of String can *ever* be an instance of `Cloneable`. It&#39;s stopping you from thinking you&#39;ve got a meaningful test when actually it will always print &quot;false&quot;.

I am trying to understand how to use reference parameters. There are several examples in my text, however they are too complicated for me to understand why and how to use them.

How and why would you want to use a reference? What would happen if you didn&#39;t make the parameter a reference, but instead left the `&amp;` off?

For example, what&#39;s the difference between these functions:

    int doSomething(int&amp; a, int&amp; b);
    int doSomething(int a, int b);

I understand that reference variables are used in order to change a formal-&gt;reference, which then allows a two-way exchange of parameters. However, that is the extent of my knowledge, and a more concrete example would be of much help. 

How do I use Reference Parameters in C++?

This is admittedly a rather loose question. My current understanding of singletons is that they are a class that you set up in such a way that only one instance is ever created.

This sounds a lot like a static class to me. The main difference being that with a static class you don&#39;t / can&#39;t instance it, you just use it such as `Math.pi()`. With a singleton class, you would still need to do something like

    singleton firstSingleton = new singleton();
    firstSingleton.set_name(&quot;foo&quot;);

    singleton secondSingleton = new singleton();

Correct me if i am wrong, but `firstSingleton == secondSingleton` right now, yes?

    secondSingleston.set_name(&quot;bar&quot;);
    firstSingleton.report_name(); // will output &quot;bar&quot; won&#39;t it?

Please note, I am asking this language independently, more about the concept. So I am not worried about actually how to code such a class, but more why you would wan&#39;t to and what thing you would need to consider.

Purpose of singletons in programming

The following compiles fine:

      Object o = new Object();
      System.out.println(o instanceof Cloneable);

But this doesn&#39;t:

      String s = new String();
      System.out.println(s instanceof Cloneable);

A compiler error is thrown.

What is the problem?

instanceof - incompatible conditional operand types

The following compiles fine:
<pre><code class="hljs language-ini"> Object o = new Object();
 System.out.println(o instanceof Cloneable);
</code></pre>
But this doesn't:
<pre><code class="hljs language-ini"> String s = new String();
 System.out.println(s instanceof Cloneable);
</code></pre>
A compiler error is thrown.
What is the problem?

How do I convert a [list][1] of String into an [array][2]? The following code returns an error.

	public static void main(String[] args) {
		List&lt;String&gt; strlist = new ArrayList&lt;String&gt;();
		strlist.add(&quot;sdfs1&quot;);
		strlist.add(&quot;sdfs2&quot;);
		String[] strarray = (String[]) strlist.toArray();		
		System.out.println(strarray);
	}
Error:

    Exception in thread &quot;main&quot; java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
    	at test.main(test.java:10)


  [1]: https://docs.oracle.com/javase/8/docs/api/java/util/List.html
  [2]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html

Converting List&lt;String&gt; to String[] in Java

I have a program which i have myself written in java, but I want to test method execution times and get timings for specific methods. I was wondering if this is possible, by maybe somehow an eclipse plug-in? or maybe inserting some code?

	
I see, it is quite a small program, nothing more than 1500 lines, which would be better a dedicated tool or `System.currentTimeMillis()`?

How can i benchmark method execution time in java?

A friend of mine was asked the following question today at interview for the position of software developer:

Given two string `s1` and `s2` how will you check if `s1` is a **rotated** version of `s2` ?

***Example:*** 

If `s1 = &quot;stackoverflow&quot;` then the following are some of its rotated versions:

    &quot;tackoverflows&quot;
    &quot;ackoverflowst&quot;
    &quot;overflowstack&quot;

where as `&quot;stackoverflwo&quot;` is ***not*** a rotated version.

The answer he gave was:&lt;br&gt;
&gt;Take `s2` and find the longest prefix that is a sub string of `s1`, that will give you the point of rotation. Once you find that point, break `s2` at that point to get `s2a` and `s2b`, then just check if `concatenate(s2a,s2b) == s1`

It looks like a good solution to me and my friend. But the interviewer thought otherwise. He asked for a simpler solution. Please help me by telling how would you do this in `Java/C/C++` ?

Thanks in advance.

Interview question: Check if one string is a rotation of other string

To continue to question further I&#39;m more interested in blogs, websites who once in a while release a tutorial, tip or best-practice on the topics I mentioned. For ex : 
http://net.tutsplus.com/ is very good website to follow if you wanna learn about or upgrade your knowledge about CSS, HTML, Javascript, PHP .. Is there a website like this for Java and related technologies?

what are good blogs to read relating java, spring, hibernate, maven?

Assuming you&#39;re already using the [m2eclipse plugin][1], what can you do when it doesn&#39;t update the dependencies to the latest in your repo?

For example, on the command line, you can just add the `-U` flag as in

    mvn clean install -U 

…to force the dependencies to be updated. Is there something like this within Eclipse? (It doesn&#39;t always seem to pick up the latest updates.)

  [1]: http://m2eclipse.sonatype.org/

How to update maven repository in Eclipse?

Why doesn&#39;t this code compile?

    	public boolean isOf(Class clazz, Object obj){
    		if(obj instanceof clazz){
      			return true;
    		}else{
    			return false;
    		}
    	}

Why I can&#39;t pass a class variable to `instanceof`?


Why can&#39;t a &quot;Class&quot; variable be passed to instanceof?

How can I see in Java if an Object is an array without using reflection?
And how can I iterate through all items without using reflection?

I use Google GWT so I am not allowed to use reflection :(

I would love to implement the following methods without using refelection:

    private boolean isArray(final Object obj) {
      //??..
    }
    
    private String toString(final Object arrayObject) {
      //??..
    }

BTW: neither do I want to use JavaScript such that I can use it in non-GWT environments.

How to see if an object is an array without using reflection?

Over the years, I&#39;ve tried to avoid `instanceof` whenever possible. Using polymorphism or the visitor pattern where applicable. I suppose it simply eases maintenance in some situations... Are there any other drawbacks that one should be aware of?

I do however see it here and there in the Java libraries so I suppose it has its place? Under what circumstances is it preferable? Is it ever unavoidable?

Is instanceof considered bad practice? If so, under what circumstances is instanceof still preferable?

Having a chain of &quot;instanceof&quot; operations is considered a &quot;code smell&quot;.  The standard answer is &quot;use polymorphism&quot;.  How would I do it in this case?

There are a number of subclasses of a base class; none of them are under my control.  An analogous situation would be with the Java classes Integer, Double, BigDecimal etc.

    if (obj instanceof Integer) {NumberStuff.handle((Integer)obj);}
    else if (obj instanceof BigDecimal) {BigDecimalStuff.handle((BigDecimal)obj);}
    else if (obj instanceof Double) {DoubleStuff.handle((Double)obj);}

I do have control over NumberStuff and so on.

I don&#39;t want to use many lines of code where a few lines would do.  (Sometimes I make a HashMap mapping Integer.class to an instance of IntegerStuff, BigDecimal.class to an instance of BigDecimalStuff etc.  But today I want something simpler.)

I&#39;d like something as simple as this:

    public static handle(Integer num) { ... }
    public static handle(BigDecimal num) { ... }

But Java just doesn&#39;t work that way.

I&#39;d like to use static methods when formatting.  The things I&#39;m formatting are composite, where a Thing1 can contain an array Thing2s and a Thing2 can contain an array of Thing1s.  I had a problem when I implemented my formatters like this:

    class Thing1Formatter {
      private static Thing2Formatter thing2Formatter = new Thing2Formatter();
      public format(Thing thing) {
          thing2Formatter.format(thing.innerThing2);
      }
    }
    class Thing2Formatter {
      private static Thing1Formatter thing1Formatter = new Thing1Formatter();
      public format(Thing2 thing) {
          thing1Formatter.format(thing.innerThing1);
      }
    }

Yes, I know the HashMap and a bit more code can fix that too.  But the &quot;instanceof&quot; seems so readable and maintainable by comparison.  Is there anything simple but not smelly?

Note added 5/10/2010:

It turns out that new subclasses will probably be added in the future, and my existing code will have to handle them gracefully.  The HashMap on Class won&#39;t work in that case because the Class won&#39;t be found.  A chain of if statements, starting with the most specific and ending with the most general, is probably the best after all:

    if (obj instanceof SubClass1) {
        // Handle all the methods and properties of SubClass1
    } else if (obj instanceof SubClass2) {
        // Handle all the methods and properties of SubClass2
    } else if (obj instanceof Interface3) {
        // Unknown class but it implements Interface3
        // so handle those methods and properties
    } else if (obj instanceof Interface4) {
        // likewise.  May want to also handle case of
        // object that implements both interfaces.
    } else {
        // New (unknown) subclass; do what I can with the base class
    }


Avoiding instanceof in Java

How to I test if `a` is a subclass of `b`?

    Class&lt;?&gt; a = A.class;
    Class&lt;?&gt; b = B.class;



How to test if one java class extends another at runtime?

I would like to know the following:

 1. `Cloneable` means we can have a clone or a copy of objects, by
    implementing the `Cloneable` interface. What are the advantages and
    disadvantages of doing that?  
 2. How does the recursive cloning happen if the object is a
    composite object?


How does Cloneable work in Java and how do I use it?

Question:

    package GoodQuestions;
    public class MyClass {	
    	MyClass() throws CloneNotSupportedException {
    		try {
    			throw new CloneNotSupportedException();
    		} catch(Exception e) {
    			e.printStackTrace();
    		}
    	}	
    
    	public static void main(String[] args) {	
    		try {
    			MyClass  obj = new MyClass();
    			MyClass obj3 = (MyClass)obj.clone();			
    		} catch (CloneNotSupportedException e) {
    			e.printStackTrace();
    		}
    	}
    }

Here class &#39;MyClass&#39; can able to clone its own object by calling the clone method in &#39;Object&#39; class.
When I try to clone the of this class(&#39;MyClass&#39;) in another class(&#39;TestSingleTon&#39;) in the same package &#39;GoodQuestions&#39; it is throwing the following compile time error.

***&#39;The method clone() from the type Object is not visible***&#39;

So here is the code it throwing the above error?

    package GoodQuestions;
    public class TestSingleTon {
    	public static void main(String[] args) {
    		MyClass  obj = new MyClass();
    		MyClass obj3 = obj.clone(); ---&gt; here is the compile error.
    	}
    }

The method clone() from object is not visible?

It is commonly understood that `Cloneable` interface in Java is broken. There are many reasons for this, which I will not mention; [others][1] already did it. It is also the position of [Java architects](http://www.artima.com/intv/bloch13.html) themselves.

My question is therefore: why has is not been deprecated yet? If the core Java team have decided that it is broken, then they must also have considered deprecation. What are their reasons against doing so (in Java 8 it is [still not deprecated][2])? 


  [1]: http://howtodoinjava.com/2012/11/10/cloneable-interface-is-broken-in-java/
  [2]: http://docs.oracle.com/javase/8/docs/api/java/lang/Cloneable.html
  [3]: http://www.artima.com/intv/bloch13.html

Why is Cloneable not deprecated?

I wrote a program that has the trait `Animal` and the struct `Dog` implementing the trait. It also has a struct `AnimalHouse` storing an animal as a trait object `Box&lt;Animal&gt;`.

    
    trait Animal {
        fn speak(&amp;self);
    }
    
    struct Dog {
        name: String,
    }
    
    impl Dog {
        fn new(name: &amp;str) -&gt; Dog {
            return Dog {
                name: name.to_string(),
            };
        }
    }
    
    impl Animal for Dog {
        fn speak(&amp;self) {
            println!{&quot;{}: ruff, ruff!&quot;, self.name};
        }
    }
    
    struct AnimalHouse {
        animal: Box&lt;Animal&gt;,
    }
    
    fn main() {
        let house = AnimalHouse {
            animal: Box::new(Dog::new(&quot;Bobby&quot;)),
        };
        house.animal.speak();
    }

It returns &quot;Bobby: ruff, ruff!&quot; as expected, but if I try to clone `house` the compiler returns errors:

    fn main() {
        let house = AnimalHouse {
            animal: Box::new(Dog::new(&quot;Bobby&quot;)),
        };
        let house2 = house.clone();
        house2.animal.speak();
    }

&lt;!-- language: none --&gt;

    error[E0599]: no method named `clone` found for type `AnimalHouse` in the current scope
      --&gt; src/main.rs:31:24
       |
    23 | struct AnimalHouse {
       | ------------------ method `clone` not found for this
    ...
    31 |     let house2 = house.clone();
       |                        ^^^^^
       |
       = help: items from traits can only be used if the trait is implemented and in scope
       = note: the following trait defines an item `clone`, perhaps you need to implement it:
               candidate #1: `std::clone::Clone`

I tried to add `#[derive(Clone)]` before `struct AnimalHouse` and got another error:

&lt;!-- language: none --&gt;

    error[E0277]: the trait bound `Animal: std::clone::Clone` is not satisfied
      --&gt; src/main.rs:25:5
       |
    25 |     animal: Box&lt;Animal&gt;,
       |     ^^^^^^^^^^^^^^^^^^^ the trait `std::clone::Clone` is not implemented for `Animal`
       |
       = note: required because of the requirements on the impl of `std::clone::Clone` for `std::boxed::Box&lt;Animal&gt;`
       = note: required by `std::clone::Clone::clone`

How do I make the struct `AnimalHouse` cloneable? Is it idiomatic Rust to use a trait object actively, in general?

Content Type	Original Author	Original Content on Stackoverflow
Question	java_geek	View Question on Stackoverflow
Solution 1 - Java	Some Guy	View Answer on Stackoverflow
Solution 2 - Java	polygenelubricants	View Answer on Stackoverflow
Solution 3 - Java	Jon Skeet	View Answer on Stackoverflow

instanceof - incompatible conditional operand types

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Solution 3 - Java

Purpose of singletons in programming

How do I use Reference Parameters in C++?

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