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Passing arguments by reference or by value

From: Java Essential Training

Video: Passing arguments by reference or by value

One of the questions you have to answer when you learn a new programming language is whether arguments passed into function are being passed by copy or by reference? Let's define these terms, when you pass arguments by copy that means that you're passing a variable in as an argument to the function or method. And within the function there is a new copy of that value, that is the value outside the function and the value inside the function are separated and any changes you make inside the function, won't be reflected in the original value.

Passing arguments by reference or by value

One of the questions you have to answer when you learn a new programming language is whether arguments passed into function are being passed by copy or by reference? Let's define these terms, when you pass arguments by copy that means that you're passing a variable in as an argument to the function or method. And within the function there is a new copy of that value, that is the value outside the function and the value inside the function are separated and any changes you make inside the function, won't be reflected in the original value.

When you pass by reference, the variable outside the function and the variable inside the function are the same variable, and any changes that you make inside the function would be reflected in the original value, once the function is completed. In Java you always are passing arguments by copy; that is you're always creating a new instance of the value inside the function. But there are certain behaviors that can make you think you're passing by reference. I'm going to show you three scenarios and explain how each of them proves the point that arguments are passed by copy.

Let's take the simplest scenario, passing primitive values. When you pass a primitive value into a function, a copy of the original value is made and any changes inside the function won't be reflected in the original. Here is a bit of code, a function named incrementValue, it receives an argument data typed as an int, within the function it increments the value by 1 and outputs the new value. Now when I call the function I am starting off with an original value, an int of 10, and outputting that value and that will clearly output the value of 10.

Then I call increment value and that will obviously output a value of 11, but what will be the final line of code output? Will it be 10 or 11? Well, because we are making a copy inside the function and not referencing the original, you'll see a value of 10 in the final output. That is to say the original value did not change. With primitives it's very easy to see that you're passing parameters by copy. Now you'll hear some Java developers say that when you pass primitives, you're making copies, but when you pass complex objects, you're actually passing by reference.

That's not the case. You're still passing by copy, but references to values inside the complex object are parts of both versions of the variable, both the original and the version inside the function. Let's take look at an example using an array. In this version of the increment value function, I'm receiving an array of integer values. Within the function I'm incrementing the first item in the array by 1 and then I'm outputting that value. In the calling code I am starting off with an array of 3 values 10, 20 and 30, then I am outputting the value of the first item in the array, once again outputting 10.

I'm calling the function and then outputting the original after. This time the behavior will be different than in the first scenario with a simple primitive, the original value is 10, inside the function it's 11, but the original value after is 11. Well, if we were passing by copy, what's going on? Here is a good way to visualize it. The variables inside a complex object are references, but when you pass the complex object to a function, you're making a copy of the original object. But both the original object and the new object are pointing to the same values internally.

A good way to say it is that a reference variable points to a location in memory and when you pass a variable to the function, a new reference is always created, but the original value is still in the same place. Let's once again take this integer array named original. It has three values of 10, 20 and 30. Let's visualize these values as locations in memory. In the original array, item 1 pointed to this location in memory, when we take that array and we pass it to a function, we're making a copy of the array, but the internal references are pointing to the same place.

So our copy of the array inside the function is still pointing to those locations in memory, those values of 10, 20 and 30. So this is why it can look like complex objects are being passed by reference. The complex objects themselves are being copied, but the internal references are being retained. Finally, let's look at a scenario with strings. I've said many times that strings are complex objects, that they contain an array of char or character values. But I've also said that strings are immutable, once you create a string, you can't change its value.

It may appear that you're changing its value, but you are actually creating a new string. Let's see what happens when you pass a string as a parameter. I've created a method named changeString, which accepts a string argument. Within the function I set its value to a string of new, and output that value. When I call the code, I start off with an original string, I output its value, I call the method and I output the original value again. With strings you'll see that you're always passing by copy, and the new copy can't reference the internal characters, because strings are immutable, they can't change once they're declared.

And so you can't reach back and change that original value, the result with a string will look like a primitive. The original will say whatever it did, inside the function it will have that value, but the original value will remain untouched. As long as you remember that in Java you're always passing parameters by copy. But with complex objects other than strings, the internal references can be retained; you'll see that the behavior is completely consistent across the entire language.

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This video is part of

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Java Essential Training

71 video lessons · 68871 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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