import org.testng.annotations.DataProvider;
import org.testng.annotations.Test;

public class DataProviderDemo {
    /**
     * always use name attribute for your dataProvider(), you can have
     * N. no of dataprovider in your Test class and with the help of dataprovider attribute of @Test(refer to line 57)
     * you can specify which dataprovider method to use.
     */

    // always returns 2D Object array
    @DataProvider(name = "testData")
    public Object[][] dataProvider(){
        // 2D Object array initialized
        Object[][] objectArray = new Object[2][2];

        /**
         * 2D Array structure
         *      |   abc     |   cde     |
         *      |   efg     |   hij     |
         *
         * what @data provider will do here is that it will send data of one row at a time to test method,
         *  then next row and so on.
         *
         *  no. of rows = no. of times your @Test will get executed
         */

        // adding value in objectArray
        objectArray[0][0] = "abc";
        objectArray[0][1] = "cde";
        objectArray[1][0] = "efg";
        objectArray[1][1] = "hij";

        // return 2D Object array
        return objectArray;
    }

    /**
     * make sure to use dataProvider attribute on your @Test annotation(refer to line 57),
     *
     * Also change you method signature according to what you are passing from your dataProvider,
     * if you are not sure on what you should do then you can also keep your parameter type as String &
     * testng would typecase it for you to string.
     *
     * no. of parameters on @test method should be same as no. of Object defined on your object array(column-wise).
     * ref line 19, we have 2 columns and 2 rows. so you should have 2 parmaters(2 columns) in your method signature.
     * (ref line no. 58)
     * -------------------------OR---------------------------
     *
     * use (String... args), here dont have to specify each parameter,
     * you will get one array which is args and you can work with it with the help of index,
     * each parameter will be stored on one index
     * assuming current case your array will have 2 indexes [0 & 1]
     * where 0th index would hold first value and 1st index would hold second value.
     * (ref line no. 63)
     */
    @Test(dataProvider = "testData") // line 57
    public void testMethod(String val1, String val2){ //line 58
        System.out.println("testMethod called with ["+val1+", "+val2+"]");
    }

    @Test(dataProvider = "testData")
    public void testMethod1(String... args){ //line 63
        System.out.println("testMethod1 called with ["+args[0]+", "+args[1]+"]");
    }
}

testMethod called with [abc, cde]
testMethod called with [efg, hij]
testMethod1 called with [abc, cde]
testMethod1 called with [efg, hij]

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="Suite">
    <test name="Test1">
        <parameter name="value1" value="abc"></parameter>
        <parameter name="value2" value="cde"></parameter>
        <parameter name="value3" value="klm"></parameter>
        <classes>
            <class name="ParametersDemo" ></class>
        </classes>
    </test> <!-- Test -->
    <test name="Test2">
        <parameter name="value1" value="efg"></parameter>
        <parameter name="value2" value="hij"></parameter>
        <classes>
            <class name="ParametersDemo" ></class>
        </classes>
    </test> <!-- Test -->
</suite> <!-- Suite -->

import org.testng.annotations.Optional;
import org.testng.annotations.Parameters;
import org.testng.annotations.Test;

public class ParametersDemo {
    /**
     *
     *  in @Parameters you should specify parameter name
     *  and same should be present in testng.xml.
     *  refer to line 5 & 13 of testng.xml
     *
     * @param val1 parameter should be specified in test.xml
     * @param val2 parameter should be specified in test.xml
     * @param val3 is Optional, if specified well it would take specified value else defualt value
     *             refer to line 15, we have use @Optional
     */
    @Test
    @Parameters({"value1","value2","value3"})
    public void testMethod(String val1,String val2,@Optional("default") String val3){
        System.out.println("testMethod called with ["+val1+","+val2+","+val3+"]");
    }
}

testMethod called with [abc,cde,klm]
testMethod called with [efg,hij,default]

testMethod called with [abc,cde,klm]
org.testng.TestNGException: 
Parameter 'value3' is required by @Test on method testMethod but has not been marked @Optional or defined