How do I fix a memory leak in java

I'm currently trying to write a program that get a stringified json object from steam and uses the object to determine if I can buy an item on the steam market or not.

It works, however I seem to be getting massive memory leaks and I have no idea how to solve this problem as I'm a beginner programmer. Here is the code:

import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.Scanner;


public class SteamMarketAlert {

    @SuppressWarnings("unused")
    private JAlertWindow alert;
    private URL jsonUrl;
    private float walletValue;
    private boolean itemBuyable;

    public SteamMarketAlert(URL itemUrl, float walletValue)
    {
        this.itemBuyable = false;
        this.jsonUrl = getJSONurl(itemUrl);
        this.walletValue = walletValue;
    }

    private URL getJSONurl(URL itemUrl)
    {
        String jsonString = itemUrl.toString();

        String firstPart = jsonString.substring(0, jsonString.indexOf("market/") + "market/".length());

        String appid = jsonString.split("/")[5];
        String marketHashName = jsonString.split("/")[6];

        String secondPart = "priceoverview/?currency=2&appid=" + appid + "&market_hash_name=" + marketHashName;

        try {
            return new URL(firstPart + secondPart);
        } catch (MalformedURLException e) {
            System.err.println("Failed to create json url");
            return null;
        }

    }

    public void checkMarket()
    {
        Thread thread = new Thread(){
            @Override
            public void run(){
                try {
                    while(!itemBuyable)
                    {
                        sleep(5000);                        
                        if(isBuyable(getPagehtml()))
                            itemBuyable = true;
                    }
                    alert = new JAlertWindow();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }   
        };

        thread.start();


    }

    private boolean isBuyable(String pagehtml)
    {
        int firstIndex = pagehtml.indexOf(";") +1;


        float marketValue = Float.parseFloat(pagehtml.substring(firstIndex, firstIndex + pagehtml.substring(firstIndex, pagehtml.length()).indexOf("\"")));

        return (marketValue <= walletValue)? true:false;
    }

    private String getPagehtml(){

        try(Scanner scanner = new Scanner(jsonUrl.openConnection().getInputStream())) {

            scanner.useDelimiter("\\Z");
            return scanner.next();
        } catch (IOException e) {               
            e.printStackTrace();
            return null;
        }

    }

    public static void main(String[] args)
    {
        try {
            float walletValue =  82.64f;
            URL itemUrl = new     URL("http://steamcommunity.com/market/listings/730/StatTrak%E2%84%A2%20P90%20%7C%20Asiimov%20%28Factory%20New%29");
            SteamMarketAlert sma = new SteamMarketAlert(itemUrl,walletValue);
            sma.checkMarket();


        } catch (MalformedURLException e) {

            e.printStackTrace();
        }

    }
}

I've narrowed the problem down to the checkMarket() method. However, I can't seem to figure out what is going on. Can you please point out how I can fix this (and possibly point out all the flaws in my code), Note the JAlertWindow object just displays a JFrame with "CAN BUY" on it - nothing special.

edit: I updated the code since posting and users informed me that try-with-resource blocks existed. Thank you to everyone who has helped me understand how java garbage collection works. :)!