Custom getters and setters in Kotlin - Step-by-Step Execution

Concept Flow - Custom getters and setters

Property declared

↓

Getter called?

No→Return stored value

Yes↓

Execute custom getter code

↓

Setter called?

No→No change

Yes↓

Execute custom setter code

↓

Update stored value if setter allows

↓

End

When you access a property, Kotlin runs the getter code. When you assign a value, Kotlin runs the setter code. Custom code can control how values are read or changed.

Execution Sample

Kotlin

var age: Int = 0
    get() = field
    set(value) {
        if (value >= 0) field = value
    }

Defines an age property with a custom setter that only accepts non-negative values.

Execution Table

Step	Action	Input/Value	Getter Called?	Setter Called?	Stored Value (field)	Output/Result
1	Initialize age	0	No	No	0	0
2	Set age to 10	10	No	Yes	10	10
3	Get age	N/A	Yes	No	10	10
4	Set age to -5	-5	No	Yes	10	Setter ignores negative, value unchanged
5	Get age	N/A	Yes	No	10	10
6	Set age to 20	20	No	Yes	20	20
7	Get age	N/A	Yes	No	20	20

💡 Execution ends after last get; setter ignores invalid negative value, so stored value stays valid.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
age (field)	0	0	10	10	10	10	20	20

Key Moments - 2 Insights

Why does the stored value not change when setting age to -5?

Because the custom setter checks if the value is negative and ignores it if so, as shown in step 4 of the execution_table where setter is called but stored value remains 10.

When is the getter code executed?

The getter runs whenever the property is read, like in steps 3, 5, and 7 where 'Getter Called?' is Yes and the stored value is returned.

Visual Quiz - 3 Questions

Look at the execution_table, what is the stored value of age after step 4?

A 0

B 10

C -5

D 20

💡 Hint

Check the 'Stored Value (field)' column at step 4 where setter ignores negative value.

At which step does the getter first return the value 10?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at 'Getter Called?' and 'Output/Result' columns in execution_table.

If the setter did not check for negative values, what would be the stored value after step 4?

A 0

B 10

C -5

D 20

💡 Hint

Consider what happens if setter always assigns the input value without condition.

Concept Snapshot

Custom getters and setters in Kotlin let you run code when reading or writing a property.
Use 'get()' to customize reading, 'set(value)' to customize writing.
Inside setter, use 'field' to access the actual stored value.
You can add checks or transformations before changing the value.
This helps keep data valid and control property behavior.

Full Transcript

This example shows a Kotlin property 'age' with custom getter and setter. The getter simply returns the stored value. The setter only updates the stored value if the new value is zero or positive. The execution table traces initializing age to 0, setting it to 10, reading it, trying to set it to -5 (which is ignored), reading again, setting to 20, and reading again. The variable tracker shows how the stored value changes only when valid values are set. Key moments clarify why negative values are ignored and when the getter runs. The quiz tests understanding of these steps. This helps beginners see how custom getters and setters control property access and assignment in Kotlin.