Develop Reference

When to use 'java.util.Objects.*'?

I was going through Java 7 features and they talked about java.util.Objects class.
What I am failing to understand is what is the functional difference betwee
java.util.Objects.toString(foo)
vs
foo == null ? "":foo.toString()
All I could see extra was a null check and functional notation instead of OOP style.
What am I missing ?

The main advantage of java.util.Objects.toString() is that you can easily use it on a return value that might be null, rather than needing to create a new local variable (or worse calling the function twice).
Compare
Foo f = getFoo();
String foo = (f==null) ? "null" : f.toString();
or the cringe-worthy and bug inducing
String foo = (getFoo()==null) ? "null" : getFoo().toString()
to the Objects.toString based version
String foo = Objects.toString(getFoo());

Calling Objects.toString(foo) just removes the need for you to remember to do null checks, and means you can use it directly on a method return value (e.g. Objects.toString(getPossibleNullObject())) without storing it in a variable first (or calling the method twice).
Note however that the method actually returns:
the result of calling toString for a non-null argument and "null" for a null argument
so it is actually equivalent to:
foo == null ? "null" : foo.toString();
if you want "" for a null value you can use the overload that passes a nullDefault return value Objects.toString(foo, "")

How to prevent null check before equals

I find stuff like this rather annoying and ugly in equals methods:
if (field == null)
{
if (other.field != null)
return false;
}
else if ( ! field.equals(other.field))
return false;
In C# I could've done this:
if( ! Object.Equals(field, other.field))
return false;
Is there something similar in Java, or what is the preferred way to do this kind if thing?

Java 7 offers java.util.Objects.equals.

Use commons-lang:
org.apache.commons.lang.ObjectUtils.equals(Object object1, Object object2)
Source code:
public static boolean equals(Object object1, Object object2) {
if (object1 == object2) {
return true;
}
if ((object1 == null) || (object2 == null)) {
return false;
}
return object1.equals(object2);
}
From Apache
http://commons.apache.org/lang/
That's about equivalent to what you do in C#

Guava equal which does this :
public static boolean equal(#Nullable Object a, #Nullable Object b) {
return a == b || (a != null && a.equals(b));
}
or null object pattern
Guava also has the somewhat related comparison chain and a load of other goodies.

I would write it this way:
return field != null && other.field != null && field.equals(other.field);
which is not as elegant as the C# code line, but much shorter then the if tree you posted.

I accept all answers technically. Practically I will not use any of them in code I have under control because all provided solutions are working around the core problem: null-values. KEEP YOUR CORE MODEL FREE FROM NULL VALUES, and the question is obsolete in this case.
At system borders like third party libraries one has to deal with null values sometimes. They should converted into meaningful values for the core model. There the given solutions are helpful.
Even if Oracle recommends the equals-Methods to be null-safe, think about that: Once you accept null values your model it is getting fragile. The equals-method will not be the last method where you will check for null. You have to manage null-checks in your method call hierarchy. Methods may not be reusable out of the box anymore. Soon, every parameter will be checked for null.
I saw both sides:
On one side code full of null checks, methods that trust not a single parameter anymore and developers that are afraid to forget a null check.
On the other side code with full expressive statements that make clear assertions to have full functioning objects that can be used without fear of NullPointerExceptions.

As part of the Project Coin, there was a proposal for adding a series of null-safe operators to Java. Sadly, they didn't make it into Java 7, maybe they'll appear in Java 8. Here is the general idea of how they would work

Actually everyone follows there own way to do this and also i would like to introduce groovy here.
There is one way
field == null ? false : true; // So basically it will return true when it is not null.
In groovy there is null safe operator for objects. Lets take an example for class
A {
String name = "test1"
String surName = "test2"
public String returnName() {
return name + surName
}
}
A a = null
a?.name
// Mentioned operator ? will actually check whether a is null or not. then it will invoke name.
Note: i didn't applied semi colon in code as this is not require in groovy.

String.valueOf() will solve some of those problems if the toString is implemented for your classes. It will spit out the toString() answer or "null" if the pointer is null.

Use == operator when you are checking for object references, if both the references refers same object it will return true. Otherwise if you are looking for object content then go with .equals method of objects.
So null means it doesn't have any memory location given in heap. So it can be simply checked with '==' operator.

Looking through an array for an empty string

An array of Strings, names, has been declared and initialized. Write the statements needed to determine whether any of the the array elements are null or refer to the empty String. Set the variable hasEmpty to true if any elements are null or empty-- otherwise set it to false.
hasEmpty=false;
for (int i=0;i<names.length;i++)
if (names[i].trim().equals("") || names[i]==null)
hasEmpty=true;
Whats wrong with my code?

Calling trim() first will result in a NullPointerException should a member of the array be null. Reverse the order of the conditions - the short-circuiting nature of || will then ensure that trim is only called on a real String object.

Consider names[i].trim().
When names[i] is a String, you really have something like someString.trim() which works fine.
When names[i] is a null, however, you really have something like null.trim(). You've already discovered that null doesn't allow a trim() method. (In fact, I'm not even really sure what 'null' is.)
Therefore, you must check for null before you invoke trim().
When you have a && b, where a and b are expressions, the checks are made left-to-right and the parser stops as soon as the issue is settled. So for the logical and operator (&&), if a is false then b is never checked. This is what allows
if (a != null && a.trim().length() > 0) { ... }
to work. if a is null, the a.trim() part is not executed since it would be pointless from a logical point of view; the value of the conditional has been decided.
Similarly for
if (a == null || a.trim().length() == 0) { ... }
if a is null then the a.trim() part is never performed and we don't get an error.

You can use the Apache Commons Lang's isBlank() to check a String:
if (StringUtils.isBlank(names[i]) {
...
}
StringUtils.isBlank is checking if the String is null or empty (i.e. if it is equals to "" when all blank characters are removed).

It's throwing a Null Pointer Exception because you're trying to run a method on a null object:
if (names[i].trim().equals("") || names[i]==null)
So, anytime that names[] has ONE name that's null, it will throw the exception. One way to solve the problem is to switch the boolean statements in this if statement:
if (names[i]==null || names[i].trim().equals(""))

return nothing , using array == null or array.length == 0?

Supposing I have a function with following signature:
Foo[] getSomeFoos()
{
//return null --- option A
or
//return new Foo[0]; --- option B
}
What is the recommended practice to return the value indicating there is no elements returned? Any pros or cons for each option?

If your method GetSomeFoos() actually 'found' 0 elements, then it should return new Foo[0].
If there was some sort of error, you should throw an Exception.
The reason is because users shouldn't have to look for nulls:
for (Foo f: GetSomeFoos()) {
// do stuff
}
In the above case, if GetSomeFoos returned null, a user will have to deal with an NullPointerException. But, if you return an empty array, the code will never enter the loop, and the user will not have to deal with an exception.

Whether to return null or empty object entirely depends on the method usage. If this method is called by a client and exposed as an API then throwing exception is better , but if it is used within ur package and never used by client then returning null is ok.

Returning null is more efficient, since you avoid the overhead of creating a new array or collection object. It's true that you then have to add a null check at the point where you use the function's output, but the overhead for that (in Java and almost all languages) is negligible. I also think that checking for null is a good habit to get into, especially when dealing with third-party libraries.
The downside is that you make the code more verbose.
If you do return an empty array, you can mitigate the performance hit by re-using the same object. Create an immutable constant, and return that:
private static final String[] EMPTY = {};
Or:
private static final List<String> EMPTY =
Collections.unmodifiableList(new ArrayList<String>());

I prefer zero-size array.
It's safer(avoid NPE) and easier(no need null check).

From Effective Java (2nd ed) - item 43: return empty arrays or collections, not nulls
In summary, there is no reason ever to return null from an array- or collection-valued method instead of returning an empty array or collection

Is it a bad idea if equals(null) throws NullPointerException instead?

The contract of equals with regards to null, is as follows:
For any non-null reference value x, x.equals(null) should return false.
This is rather peculiar, because if o1 != null and o2 == null, then we have:
o1.equals(o2) // returns false
o2.equals(o1) // throws NullPointerException
The fact that o2.equals(o1) throws NullPointerException is a good thing, because it alerts us of programmer error. And yet, that error would not be catched if for various reasons we just switched it around to o1.equals(o2), which would just "silently fail" instead.
So the questions are:
Why is it a good idea that o1.equals(o2) should return false instead of throwing NullPointerException?
Would it be a bad idea if wherever possible we rewrite the contract so that anyObject.equals(null) always throw NullPointerException instead?
On comparison with Comparable
In contrast, this is what the Comparable contract says:
Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.
If NullPointerException is appropriate for compareTo, why isn't it for equals?
Related questions
Comparable and Comparator contract with regards to null
A purely semantical argument
These are the actual words in the Object.equals(Object obj) documentation:
Indicates whether some other object is "equal to" this one.
And what is an object?
JLS 4.3.1 Objects
An object is a class instance or an array.
The reference values (often just references) are pointers to these objects, and a special null reference, which refers to no object.
My argument from this angle is really simple.
equals tests whether some other object is "equal to" this
null reference gives no other object for the test
Therefore, equals(null) should throw NullPointerException

To the question of whether this asymmetry is inconsistent, I think not, and I refer you to this ancient Zen kōan:
Ask any man if he's as good as the next man and each will say yes.
Ask any man if he's as good as nobody and each will say no.
Ask nobody if it's as good as any man and you'll never get a reply.
At that moment, the compiler reached enlightenment.

An exception really should be an exceptional situation. A null pointer might not be a programmer error.
You quoted the existing contract. If you decide to go against convention, after all this time, when every Java developer expects equals to return false, you'll be doing something unexpected and unwelcome that will make your class a pariah.
I could't disagree more. I would not rewrite equals to throw an exception all the time. I'd replace any class that did that if I were its client.

Think of how .equals is related to == and .compareTo is related to the comparison operators >, <, >=, <=.
If you're going to argue that using .equals to compare an object to null should throw a NPE, then you'd have to say that this code should throw one as well:
Object o1 = new Object();
Object o2 = null;
boolean b = (o1 == o2); // should throw NPE here!
The difference between o1.equals(o2) and o2.equals(o1) is that in the first case you're comparing something to null, similar to o1 == o2, while in the second case, the equals method is never actually executed so there's no comparison happening at all.
Regarding the .compareTo contract, comparing a non-null object with a null object is like trying do this:
int j = 0;
if(j > null) {
...
}
Obviously this won't compile. You can use auto-unboxing to make it compile, but you get a NPE when you do the comparison, which is consistent with the .compareTo contract:
Integer i = null;
int j = 0;
if(j > i) { // NPE
...
}

Not that this is neccessarily an answer to your question, it is just an example of when I find it useful that the behaviour is how it is now.
private static final String CONSTANT_STRING = "Some value";
String text = getText(); // Whatever getText() might be, possibly returning null.
As it stands I can do.
if (CONSTANT_STRING.equals(text)) {
// do something.
}
And I have no chance of getting a NullPointerException. If it were changed as you suggested, I would be back to having to do:
if (text != null && text.equals(CONSTANT_STRING)) {
// do something.
}
Is this a good enough reason for the behaviour to be as it is?? I don't know, but it is a useful side-effect.

If you take object oriented concepts into account, and consider the whole sender and receiver roles, I'd say that behaviour is convenient. See in the first case you're asking an object if he is equal to nobody. He SHOULD say "NO, I'm not".
In the second case though, you don't have a reference to anyone So you aren't really asking anyone. THIS should throw an exception, the first case shouldn't.
I think it's only asymmetric if you kind of forget about object orientation and treat the expression as a mathematical equality. However, in this paradigm both ends play different roles, so it is to be expected that order matters.
As one final point. A null pointer exception should be raised when there's an error in your code. However, Asking an object if he is nobody, shouldn't be considered a programming flaw. I think it's perfectly ok to ask an object if he isn't null. What if you don't control the source that provides you with the object? and this source sends you null. Would you check if the object is null and only afterwards see if they are equals? Wouldn't it be more intuitive to just compare the two and whatever the second object is the comparison will be carried out without exceptions?
In all honesty, I would be pissed if an equals method within its body returns a null pointer exception on purpose. Equals is meant to be used against any sort of object, so it shouldn't be so picky on what it receives. If an equals method returned npe, the last thing on my mind would be that it did that on purpose. Specially considering it's an unchecked exception. IF you did raise an npe a guy would have to remember to always check for null before calling your method, or even worse, surround the call to equals in a try/catch block (God I hate try/catch blocks) But oh well...

Personally, I'd rather it perform as it does.
The NullPointerException identifies that the problem is in the object against which the equals operation is being performed.
If the NullPointerException was used as you suggest and you tried the (sort of pointless) operation of...
o1.equals(o1) where o1= null...
Is the NullPointerException thrown because your comparison function is screwed or because o1 is null but you didn't realise?
An extreme example, I know, but with current behaviour I feel you can tell easily where the problem lies.

In the first case o1.equals(o2) returns false because o1 is not equal to o2, which is perfectly fine. In the second case, it throws NullPointerException because o2 is null. One cannot call any method on a null. It may be a limitation of programming languages in general, but we have to live with it.
It is also not a good idea to throw NullPointerException you are violating the contract for the equals method and making things more complex than it has to be.

There are many common situations where null is not in any way exceptional, e.g. it may simply represent the (non-exceptional) case where a key has no value, or otherwise stand for “nothing”. Hence, doing x.equals(y) with an unknown y is also quite common, and having to always check for null first would be just wasted effort.
As for why null.equals(y) is different, it is a programming error to call any instance method on a null reference in Java, and therefore worthy of an exception. The ordering of x and y in x.equals(y) should be chosen such that x is known to not be null. I would argue that in almost all cases this reordering can be done based on what is known about the objects beforehand (e.g., from their origin, or by checking against null for other method calls).
Meanwhile if both objects are of unknown “nullness”, then other code almost certainly requires checking at least one of them, or not much can be done with the object without risking the NullPointerException.
And since this is the way it is specified, it is a programming error to break the contract and raise an exception for a null argument to equals. And if you consider the alternative of requiring an exception to be thrown, then every implementation of equals would have to make a special case of it, and every call to equals with any potentially null object would have to check before calling.
It could have been specified differently (i.e., the precondition of equals would require the argument to be non-null), so this is not to say that your argumentation is invalid, but the current specification makes for a simpler and more practical programming language.

Note that the contract is "for any non-null reference x". So the implementation will look like:
if (x != null) {
if (x.equals(null)) {
return false;
}
}
x need not be null to be deemed equal to null because the following definition of equals is possible:
public boolean equals(Object obj) {
// ...
// If someMember is 0 this object is considered as equal to null.
if (this.someMember == 0 and obj == null) {
return true;
}
return false;
}

I think it's about convenience and more importantly consistency - allowing nulls to be part of the comparison avoids having to do a null check and implement the semantics of that each time equals is called. null references are legal in many collection types, so it makes sense they can appear as the right side of the comparison.
Using instance methods for equality, comparison etc., necessarily makes the arrangement asymmetric - a little hassle for the huge gain of polymorphism. When I don't need polymorphism, I sometimes create a symmetric static method with two arguments, MyObject.equals(MyObjecta, MyObject b). This method then checks whether one or both arguments are null references. If I specifically want to exclude null references, then I create an additional method e.g. equalsStrict() or similar, that does a null check before delegating to the other method.

You should return false if the parameter is null.
To show that this is the standard, see 'Objects.equals(Object, Object) from java.util, that performs an assymetric null check on the first parameter only (on which equals(Object) will be called). From the OpenJDK SE 11 source code (SE 1.7 contains exactly the same):
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
This also handles two null values as equal.

This is a tricky question. For backward compatability you can't do so.
Imagine the following scenario
void m (Object o) {
if (one.equals (o)) {}
else if (two.equals (o)) {}
else {}
}
Now with equals returning false else clause will get executed, but not when throwing an exception.
Also null is not really equal to say "2" so it makes perfect sense to return false. Then it is probably better to insist null.equals("b") to return also false :))
But this requirement does make a strange and non symmetric equals relation.