How is ambiguous overloaded method call resolved in java?

How compiler resolves ambiguous method overloading call. OR
Which overloaded method will get selected for null parameter in java. OR
Important Java Interview Questions On Method Overloading OR
Method Overloading Tutorial

What is method overloading?

If a class have multiple methods with same name but with different parameters list, it is known as Method Overloading. Parameters lists should differ in either,

Number of parameters.
Data type of parameters.
Sequence of data type of parameters.

Example:

class ArithmeticOperations{  
 public void add(int num1,int num2){
  System.out.println(num1 + num2);
 }  
 
 public int add(int num1,int num2, int num3){
  int result = num1 + num2 + num3;
  return result;
 }
  
 public static void main(String args[]){  
  ArithmeticOperations obj = new ArithmeticOperations();    
  obj.add(1,2);
  int result = obj.add(1,2,3);
  System.out.println(result);
 }
}

What is advantage of method Overloading?

Method overloading increases the readability of the program.

Example of Java API using method Overloading?

1. "valueOf" method of String class is overloaded. It returns String representation of variable passed.

static String valueOf(boolean b) 
static String valueOf(char c) 
static String valueOf(char[] data) 
static String valueOf(char[] data, int offset, int count) 
static String valueOf(double d) 
static String valueOf(float f) 
static String valueOf(int i) 
static String valueOf(long l) 
static String valueOf(Object obj)

2. "substring" method of String class is overloaded. It returns a new string that is a substring of this string.

substring(beginIndex);
substring(beginIndex, endIndex);

3. "println" method of PrintStream class is overloaded. It Prints a data supplied and then terminate the line.

println(char x[]);
println(String x);
println(double x);
println(char x); 
println(Object x);

Note:
Method overloading is one of the way through which java supports polymorphism.
Polymorphishm achieved using method overloading is known as Compile time/Static polymorphism because which method will be invoked is decided at compile time.

Example of Real time use of method Overloading?

There is a Organization where many Employee works and we need to design a system for it. In design, for getting Employee we would have a EmployeeModel something like this,

class Employee{
 private int id;
 private String name;

 //Getter & Setters
}

class EmployeeModel{  

 //Get Employee by name and dob.
 public Employee getEmployee(String name, Date dob){
  //Logic for fetching Employee emp
  return emp;
 }

 //Get Employee by name
 public List<Employee> getEmployee(String name){
  //Logic for fetching list of Employee emp
  return listEmp;
 }

 //Get Employee by id  
 public Employee getEmployee(int employeeId){
  //Logic for fetching Employee emp
  return emp;
 }
}

Question 1. What is the output of below program?

public class OverloadedMethod{
 public void test(String str) {
  System.out.println("String");
 }
 
 public void test(Object obj) {
  System.out.println("Object");
 }
 
 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(null);
 }
}

Output: String

On what basis Compiler decides which method to invoke?

If more than one method is both accessible and applicable to a method invocation then
Java compiler uses the set of rules that the most specific method is chosen for invocation.

In our case above, call to test(null) is suitable for both the test method declared, So in this case most specific method is chosen for invocation.

Compiler check the class hierarchy of method parameter and whichever class is least general one that is the class which is encounter first in bottom up hierarchy, that method is invoked.

Compiler will found String class as the most general class in bottom up inheritance hierarchy, that is why test(String) method is invoked

Question 2. What is the output of below program?

public class OverloadedMethod{
 public void test(String str) {
  System.out.println("String");
 }
 
 public void test(StringBuffer obj) {
  System.out.println("Object");
 }
 
 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(null);
 }
}

Output: Compile time error:
The method test(String) is ambiguous for the type OverloadedMethod

Why compiler doesn't able to resolve overloaded method call this time?

StringBuffer and String class are both at same level in Object hierarchy, So in this case Compiler will not able to resolve which method to invoke and it gives Compile time error.

Let's see one more example and we will be get this better.

Question 3. What is the output of below program?

class A{}

class B extends A{}

class C extends B{}

class OverloadedMethod{
 public void test(B obj) {
  System.out.println("B");
 }
 
 public void test(C obj) {
  System.out.println("C");
 }
 
 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(null);
 }
}

Output: C

Output is "C" because test(null) method call maps to method which contains parameter as class which is lowest in class hierarchy.

Question 4. What is the output of below program?

class OverloadedMethod{
 public void test(String obj1, Object obj2) {
  System.out.println("B");
 }
 
 public void test(Object obj1, String obj2) {
  System.out.println("C");
 }
 
 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(null, null);
 }
}

Output: Compile time error because method contains parameter in opposite order and none of
the test method in this case is more specific and can be resolved.

Question 5. What is the output of below program?

class OverloadedMethod{
 public void test(int[] intArr) {
  System.out.println("int array");
 }
 
 public void test(char[] charArr) {
  System.out.println("char array");
 }
 
 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(null);
 }
}

Output: Compile time error.
int[] is class with name "[I" and superclass of int[] is java.lang.Object.
char[] is class with name "[C" and superclass of char[] is java.lang.Object.

So both the classes int[] and char[] are at same level to Object class and fall in different
hierarchy, so compiler will not able to resolve as which method to invoke.

Note:
int is primitive type in java but int[] is not primitive and it is class which extends Object class.
you can pass null to int[] because it is object and passing null to int will give compiler error.

You can execute below lines to get the facts,

     System.out.println(int[].class.isPrimitive());
        System.out.println(int[].class.getName());
        System.out.println(int[].class.getSuperclass());
        
        System.out.println(int.class.isPrimitive());
        System.out.println(int.class.getName());
        System.out.println(int.class.getSuperclass());

        Object ref=new int[]{1,2,3};   // valid statement?
        Object[] ref=new int[]{1,2,3}; // valid statement?

Output:
false
[I
class java.lang.Object

true
int
null

line number 9 is valid statement or not? It is perfectly valid because int[] extends Object and Object is base class.
line number 10 is invalid statement because int[] extends Object class and not Object[] class.

Question 6. What is the output of below program?

class OverloadedMethod{     
    public void test(Object obj) {
        System.out.println("Object");
    }
    public void test(char[] obj) {
        System.out.println("Char");
    } 
    public void test(Integer obj) {
        System.out.println("Int");
    } 
    public static void main(String[] args) {
        OverloadedMethod obj = new OverloadedMethod();
        obj.test(null);
    }
}

Output: Compile time error.
char[] is class with name "[C" and superclass of char[] is java.lang.Object.
Integer is class and superclass of Integer is Number --> Object.

So both the classes char[] and Integer are at same level to Object class and fall in
different hierarchy, so compiler will not able to resolve as which method to invoke.

Question 7. What is the output of below program?

class OverloadedMethod{
    public void test(long lng) {
        System.out.println("Long");
    }
     
    public void test(Integer integer) {
        System.out.println("Integer");
    }
     
    public static void main(String[] args) {
        OverloadedMethod obj = new OverloadedMethod();
        obj.test(1);
    }
}

Output: Long
Because Widening wins over Boxing.

Note: Rules that applies for evaluating method call in overloading.

Widening wins over boxing eg. test(10) will call test(long) instead of test(Integer) if both are available.
Widening wins over var-args eg test(byte,byte) will call test(int,int) instead of test(byte...x) method.
Boxing beats var-args eg test(byte,byte) will call test(Byte,Byte) instead of test(byte...x) method.
Widening of reference variable depends on inheritance tree(so, Integer cannot be widened to Long. But, Integer widened to Number because they are in same inheritance hierarchy).

You cannot widen and then box. Eg. test(int) cannot call test(Long) since to call test(Long) the compiler need to convert int to Integer then Integer to Long which is not possible.
You can box and then widen. Eg. An int can boxed to Integer and then widen to Object.
var-args can be combined with either boxing or widening.

Question 8. What is the output of below program?

class OverloadedMethod{     
    public void test(char obj) {
        System.out.println("Char");
    }
    public void test(int obj) {
        System.out.println("Int");
    } 
    public static void main(String[] args) {
        OverloadedMethod obj = new OverloadedMethod();
        obj.test('a');
    }
}

Output: Char

What will be the output if there is only test(int obj) method and comment out test(char obj) method?
No compile error and output will be "Int", because Java Data type Widening will come in picture.

Java's widening conversions rules are,

From a byte   ---> short ---> int ---> long ---> float ---> double
From a short ---> int ---> long ---> float ---> double
From a char ---> int ---> long ---> float ---> double
From an int    ---> long ---> float ---> double
From a long   ---> float ---> double
From a float   ---> double
Java's Narrow conversions rules are,

From a byte     ---> char
From a short    ---> byte ---> char
From a char     ---> byte ---> short
From an int      ---> byte ---> short ---> char
From a long     ---> byte ---> short ---> char ---> int
From a float     ---> byte ---> short ---> char ---> int ---> long
From a double ---> byte ---> short ---> char ---> int ---> long ---> float

Question 9. What is the output of below program?

class OverloadedMethod{     
    public void test(int i) {
        System.out.println("Int");
    }
    public void test(char... c) {
        System.out.println("Char varargs");
    } 
    
    public static void main(String[] args) {
        OverloadedMethod obj = new OverloadedMethod();
        obj.test('x');
        obj.test(97);
    }
}

Output:
Int
Int

Note: Methods with varargs (...) have the lowest priority.
Priority goes as Widening --> Boxing --> Var args.

Logical reason behind Var-args having least priority is varargs were added late in Java API,
Giving variable arguments a extremely low priority is also necessary for backwards-compatibility, otherwise giving high priority to variable argument will mess already written overloaded methods.

Question 10. What is the output of below program?

class OverloadedMethod{     
    public void test(Integer i) {
        System.out.println("Integer");
    }
    public void test(Long l) {
        System.out.println("Long");
    } 
    
    public static void main(String[] args) {
        OverloadedMethod obj = new OverloadedMethod();
        short s = 10;
        obj.test(s);
    }
}

Output: Compile time error. It is breaking rule "You cannot widen and then box"
For short primitive type to be resolved to either Integer or Long wrapper class, it has to
first widen short to int and then box it to Integer, which is not possible.

Question 11. What is the output of below program?

class OverloadedMethod{     
    public void test(Character c) {
        System.out.println("Character");
    }
    public void test(Integer i) {
        System.out.println("Integer");
    } 
    public void test(Object o) {
        System.out.println("Object");
    }    
    public static void main(String[] args) {
        OverloadedMethod obj = new OverloadedMethod();
        byte b = 10;
        obj.test(b);
    }
}

Output: Object
              "You cannot widen and then box" but "You can box and then widen".
               obj.test(b) can be resolved to either test(Integer i) or test(Object obj).

   For resolving to test(Integer i), It has to first widen byte to int and then box it to Integer.
   It is not permissible in java that is "You cannot widen and then box"

   For resolving to test(Object i), It has to first box to Integer and then widen to Object.
   It is permissible in java that is "You can box and then widen". and that is why output is
               "Object".

Question 12. What is the output of below program?

class OverloadedMethod{     
 public void test(char i, Character j) {
  System.out.println("method 1");
 }
 public void test(Character i, Character j) {
  System.out.println("method 2");
 } 

 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test('a','a');
 }
}

Output: Compile Error
              Why it gave Compile Error?

               Java overloaded method call is resolved using 3 steps,
               STEP 1: Compiler will try to resolve call without boxing and unboxing and variable
                               argument.

               STEP 2: Compiler will try to resolve call by using boxing and unboxing.

               STEP 3: Compiler will try to resolve call by using boxing/unboxing and variable argument.

               If call is not resolved by using any of the 3 ways then it gives compile error.

               In our case, method call, test('a','a') is not resolved by using STEP 1 because,
               In STEP 1, 1st argument of method test(char i, Character j) is mapped but for 2nd parameter
               it is not able to resolved using STEP 1, so control flow to STEP 2 for method resolution.

               In STEP 2, Compiler tries to resolve call using Boxing and Un-Boxing and found that both
               method is applicable and valid for call to test('a','a') and that is why Compiler stop here and
               gave Ambiguous method call error.

Question 13. What is the output of below program?

class OverloadedMethod{     
 public void test(long i, int j) {
  System.out.println("method 1");
 }
 public void test(int i, Integer j) {
  System.out.println("method 2");
 } 

 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(5,6);
 }
}

Output: method 1
              The first method has one parameter that requires widening and another that is exact match.
              The second method has one parameter with exact match and another that requires boxing.
              Since widening has precedence over boxing, the first method is chosen.

              Priority goes as: Exact Match > Widening > Boxing/unboxing > Varargs

Question 13. What is the output of below program?

class OverloadedMethod{     
 public void test(int... i) {
  System.out.println("method 1");
 }
 public void test(Integer... i) {
  System.out.println("method 2");
 } 

 public static void main(String[] args) {
  OverloadedMethod obj = new OverloadedMethod();
  obj.test(1);
 }
}

Output: Compile time error
              Why it gave Compile Error?

               Java overloaded method call is resolved using 3 steps,
               STEP 1: Compiler will try to resolve call without boxing and unboxing and variable
                               argument.

               STEP 2: Compiler will try to resolve call by using boxing and unboxing.

               STEP 3: Compiler will try to resolve call by using boxing/unboxing and variable argument.

               If call is not resolved by using any of the 3 ways then it gives compile error.

               In our case, method call, test(1) is not resolved by using STEP 1 because,
               In STEP 1,compiler will try data type widening and see whether call is resolved but no in
               our case.

               In STEP 2, Compiler tries to resolve call using Boxing and Un-Boxing and see whether call
               is resolved but no in our case.

               In STEP 3, Compiler tries to resolve call using Boxing and Un-Boxing along with variable
               argument and see whether call is resolved. In our case at this step compiler will find both
               method test(int...) and test(Integer...) is applicable and call resolved to 2 methods that is
               why it gave compile error.

Question 14. What is the output of below program?

class OverloadedMethod{

 public static void main(String[] args) {
           System.out.println(null);
 }
}

Output: Compile time error

              It gave compiler error because call to println(null) is ambiguous and compiler will not able
              to decide as which method to invoke among multiple overloaded println() method as none
of the class level parameter is more specific.

Java Language Specification rules for resolving overloading call passes through 3 phases,

The first phase (§15.12.2.2) performs overload resolution without permitting boxing or unboxing conversion, or the use of variable arity method invocation. If no applicable method is found during this phase then processing continues to the second phase. This guarantees that any calls that were valid in the Java programming language before Java SE 5.0 are not considered ambiguous as the result of the introduction of variable arity methods, implicit boxing and/or unboxing. However, the declaration of a variable arity method (§8.4.1) can change the method chosen for a given method method invocation expression, because a variable arity method is treated as a fixed arity method in the first phase. For example, declaring m(Object...) in a class which already declares m(Object) causes m(Object) to no longer be chosen for some invocation expressions (such as m(null)), as m(Object[]) is more specific.

The second phase (§15.12.2.3) performs overload resolution while allowing boxing and unboxing, but still precludes the use of variable arity method invocation. If no applicable method is found during this phase then processing continues to the third phase. This ensures that a method is never chosen through variable arity method invocation if it is applicable through fixed arity method invocation.

The third phase (§15.12.2.4) allows overloading to be combined with variable arity methods, boxing, and unboxing.