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In Java, the heap space is mainly used for garbage collection and allocating memory for objects.

A default heap space is allocated when JVM is installed on our machine, but it may be different. The following points show how we can increase the heap size using two methods in Java.

Increasing the Java heap size using the command line

The first way to increase the heap size in Java is to use the command line to run a command that overrides the default Java heap space. We follow these two steps.

Step 1: Check the default maximum heap size in Java

Before we change the heap size in Java, we must know the default maximum heap size allocated to the JVM. To do this, we use the code that Runtime.getRuntime().maxMemory()returns the maximum heap size in bytes.

In the program, we get the maximum heap space in bytes and convert it to a more readable format like megabytes or gigabytes. The output shows that our current default size is about 1.48 GB.

We do this step so as not to allocate a size smaller than the default size of the heap.

public class JavaExample {
  public static void main(String[] args) {
    double maxHeapSize = Runtime.getRuntime().maxMemory();
    String sizeInReadableForm;

    double kbSize = maxHeapSize / 1024;
    double mbSize = kbSize / 1024;
    double gbSize = mbSize / 1024;

    if (gbSize > 0) {
      sizeInReadableForm = gbSize + " GB";
    } else if (mbSize > 0) {
      sizeInReadableForm = mbSize + " MB";
    } else {
      sizeInReadableForm = kbSize + " KB";
    }

    System.out.println("Maximum Heap Size: " + sizeInReadableForm);
  }
}

Output:

Maximum Heap Size: 1.48046875 GB

Step 2: Set the maximum heap size using the command line

Now that we know the maximum Java heap size, we can increase it using the command line.

To get the commands we can use to modify the heap size, we open the command line and use the command which java -Xreturns a full list of commands, but we are only interested in the following two commands.

The command -Xmssets the initial and minimum heap size, while -Xmsto set the maximum size; we need to use the second command.

 -Xms<size>        set initial Java heap size
 -Xmx<size>        set maximum Java heap size

In the command line, we write the following command to run the program we saw in the first step.

We increase the heap size by 2GB using -Xmx2g. Note that we write the memory location as a single character; for example, if we want to set a 2GB heap size, we write 2g.

The output shows that the default size of approximately 1.4GB is now changed to 2.0GB.

java -Xmx2g JavaExample

Output:

Maximum Heap Size: 2.0 GB

Increase the Java heap size in the IDE

If we run the program, we can also increase the Java heap space in the IDE. Although some IDEs may have different steps to perform the task, if we follow these steps, we can modify the heap size in most IDEs.

• We find the menu item in the menu bar 运行.
• In 运行the menu items we have to turn on an option called 运行配置or .编辑配置
• A dialog box opens showing details about the application we are about to run using the configuration.

  There will be two input boxes: one for program arguments and the second for VM arguments. If there were no input boxes for VM arguments, we could Modify Optionsadd them using the button.

• In the input box of VM arguments, we write the following command to run the program with increased heap size.

  -Xmx2g