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Hello team!

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In this lesson we will discuss with you a very important topic which even some more experienced software

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engineers can't understand.

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But I will explain you this topic with simple examples and you will remember it easily.

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Today we will talk with you how data is passed to the method in Java by value or by reference.

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Why?

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This question is important because in different programming languages, you can decide whether you want

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to pass value to the method by value or by reference.

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This might affect the state of the data which was supposed to be modified after specific method call.

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So you already understand that we can pass arguments by value and by reference.

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What does pass by value mean?

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Passing by value means that the value of the method parameter is copied into another location of your

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memory, and when accessing or modifying the variable within your function, only the copy is accessed

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or modified and the original value is left untouched.

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Imagine empty cup and specific function which you want to use to fill this cup with coffee.

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In case we pass by value, we copy this cup inside the function and all changes with this cup affects

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only cup which was copied.

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Then what does pass by reference mean?

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Passing by reference means that the memory address of the variable, or it is better to say a pointer

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to the memory location is passed to the method.

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In case we pass by reference, we pass the address where this cup is located.

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And when I fill the cup inside the function, I also fill the original cup.

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And now please remember the rule in Java.

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Primitive and reference types of data are passed by value.

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That is the only right answer on the interview.

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You can read in internet that Java objects are passed by reference, and primitive types are passed

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by values.

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That is not true on the simple examples.

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I will prove you this.

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Here I have int value which is equal to ten.

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I have method which takes int as a parameter inside it.

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We want to increase I by 100.

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What value of I will be in console here?

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After the method call, the value will be ten.

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Why?

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Because in Java everything is passed by value.

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Here when I called method, the copy of I variable was created and when I increased I by 100 value,

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the result was assigned to the local I variable, but not to this one.

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These two are different.

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They remember in the lesson when we discussed Java memory model, I told you about stack memory.

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When this method is called copy of the I was created in stack.

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So in this case how I can change the value of I.

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Here the only way to do this in Java is to use return statement.

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Your message should return some value which will be assigned to the variable.

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Like in this example I call method change int, I add 100 and I return the value from this method which

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is assigned to I variable.

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Here.

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Let's move on.

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Now I have array of ints and you remember that arrays are reference type of data.

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I have method which has some logic inside to change state of the array.

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I want to make second element in this array equal to 200, but not two as in original array.

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Let me print this array to console after this method call.

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What do you think will be the second element?

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2 or 200.

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And it is 200.

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You might be wondering how this could happen if in Java we pass everything by value, then modification

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which we did in the method should not affect original array.

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Let's understand what has happened here.

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I call the method and pass reference to the array object.

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Attention.

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I pass the reference to the array, but not the array, and copy of the reference is created here.

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I can get access to the array through the reference.

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So here I use the reference to get access to the same array as here.

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These two variables reference to the same array object in the heap memory.

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Probably now you want to get proofs that this is a copy of reference, but not copy of the array.

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I have another method with name clear array.

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It takes an array as a parameter and makes this variable reference to null.

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Let me call this method with our array.

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Now I print it to console.

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What will be printed to console null or values from array?

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Values from array is on place.

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Why?

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Because this is copy of reference and now local method variable which exists only in scope of this method.

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References to null.

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No problems at all.

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This reference still refers to the object on heap memory.

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And now you can be sure that even when we pass objects in Java to the method, we still pass by value

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because we pass reference to the object and we pass value of the reference.

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Hope examples from this lesson help you understand the topic better.

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Let's recap what we have learned today.

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Today we learned what does pass by value and pass by reference mean.

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We also understood that in Java everything is passed by value.

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On the examples, we saw how primitive and reference types passed by value.

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That's all for today.

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Thanks a lot for your attention.

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See you in the next lesson.

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Your.