---
Title: JavaReader
alwaysopen: false
categories:
- docs
- operate
- stack
description: A template for creating custom readers.
linkTitle: JavaReader
weight: 60
---

The `JavaReader` is an abstract class that allows you to create a custom reader in Java.

## Create a custom reader

To create a custom reader:

- Extend the `JavaReader` class
- Override the `iterator()` function

## Custom reader example

The implementation of the [`KeysOnlyReader`]({{< relref "/operate/oss_and_stack/stack-with-enterprise/gears-v1/jvm/classes/readers/keysonlyreader" >}}) class shows how to create a custom reader with `JavaReader`:

```java
import java.util.Iterator;

import gears.GearsBuilder;

/**
 * A reader that only reads key names from the key space
 *
 */
public class KeysOnlyReader extends JavaReader<String> {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	
	private String scanSize;
	private String pattern;
	
	/**
	 * Create a new KeysOnlyReader reader
	 * @param scanSize - the size to use with the scan command
	 * @param pattern - the pattern of the keys to read
	 */
	public KeysOnlyReader(int scanSize, String pattern) {
		this.scanSize = Integer.toString(scanSize);
		this.pattern = pattern;
	}
	
	/**
	 * Create a new KeysOnlyReader reader with default pattern (*) and default
	 * scan size (10000)
	 */
	public KeysOnlyReader() {
		this(10000, "*");
	}

	@Override
	public Iterator<String> iterator() {
		return new Iterator<String>() {

			String cursor = "0";
			int currIndex = 0;
			Object[] keys = null;
			boolean isDone = false;
			String nextKey = null;
			
			private String innerNext() {
				while(!isDone) {
					if(keys == null) {
						Object[] res = (Object[]) GearsBuilder.execute("scan", 
										cursor == null ? "0" : cursor,
										"MATCH", pattern, "COUNT", scanSize);
						keys = (Object[])res[1];
						cursor = (String)res[0];
						currIndex = 0;
					}
					if(currIndex < keys.length) {
						return (String) keys[currIndex++];
					}
					
					keys = null;
					if(cursor.charAt(0) == '0') {
						isDone = true;
					}
				}
				return null;
			}
			
			@Override
			public boolean hasNext() {
				if(nextKey == null) {
					nextKey = innerNext();
				}
				return !isDone;
			}

			@Override
			public String next() {
				String temp = nextKey != null ? nextKey : innerNext();
				nextKey = innerNext();
				return temp;
			}
			
		};
	}

}
```