The LongSupplier is a functional interface in Java that is part of the java.util.function package. It represents a function that supplies a long-valued result. This interface is particularly useful when we need to generate or provide long values without requiring input arguments.

Since LongSupplier is a functional interface, it has a single abstract method:

• long getAsLong(): This method is used to return a long value.

Here is how we can use the LongSupplier interface in Java:

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Example 1: Basic Usage with Lambda Expression

package org.kodejava.util.function;

import java.util.function.LongSupplier;

public class LongSupplierExample {
    public static void main(String[] args) {
        // Using a LongSupplier to provide the current system time in milliseconds
        LongSupplier currentTimeSupplier = () -> System.currentTimeMillis();

        // Getting a long value
        long currentTime = currentTimeSupplier.getAsLong();
        System.out.println("Current Time in Milliseconds: " + currentTime);
    }
}

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Example 2: Using Method References

We can also use method references instead of a lambda if it matches the expected signature of the LongSupplier.

package org.kodejava.util.function;

import java.util.function.LongSupplier;

public class LongSupplierExample2 {
    public static void main(String[] args) {
        // Using method reference
        LongSupplier nanoTimeSupplier = System::nanoTime;

        // Getting a long value
        long nanoTime = nanoTimeSupplier.getAsLong();
        System.out.println("Current Time in Nanoseconds: " + nanoTime);
    }
}

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Example 3: Custom Implementation

We can create our own implementation of LongSupplier.

package org.kodejava.util.function;

import java.util.function.LongSupplier;

public class LongSupplierExample3 {
    public static void main(String[] args) {
        // Custom implementation
        LongSupplier randomLongSupplier = new LongSupplier() {
            @Override
            public long getAsLong() {
                // Generate a random long value
                return (long) (Math.random() * 1000);
            }
        };

        // Getting a long value
        long randomValue = randomLongSupplier.getAsLong();
        System.out.println("Random Long Value: " + randomValue);
    }
}

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Example 4: Using with Streams

LongSupplier can also be useful when used with streams for generating a sequence of long values.

package org.kodejava.util.function;

import java.util.function.LongSupplier;
import java.util.stream.LongStream;

public class LongSupplierExample4 {
    public static void main(String[] args) {
        // LongSupplier to generate an infinite series of long values
        LongSupplier supplier = new LongSupplier() {
            private long start = 1;

            @Override
            public long getAsLong() {
                return start++;
            }
        };

        // Using the LongSupplier with LongStream.generate
        LongStream.generate(supplier)
                .limit(10) // Limit to 10 values
                .forEach(System.out::println); // Print each value
    }
}

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Key Points:

1. LongSupplier is a no-argument functional interface that always returns a long value.
2. It is lightweight and simple to use with lambdas, method references, or custom implementations.
3. Useful in scenarios where we need to continuously or repeatedly generate long values, such as timestamps, counters, or random numbers.