Implement your own String replaceAll method. Write tests,

import com.citrix.mstest.Application;
import org.springframework.boot.test.SpringApplicationConfiguration;
import org.springframework.test.context.testng.AbstractTestNGSpringContextTests;
import org.testng.annotations.DataProvider;
import org.testng.annotations.Test;

import static org.testng.Assert.assertEquals;

@SpringApplicationConfiguration(Application.class)
public class Some1Test extends AbstractTestNGSpringContextTests {

    @DataProvider
    public Object[][] replaceAllParameters() {
        return new Object[][]{
                {"hello Eric Erik", "E", "H", "hello Hric Hrik"},
                {"hello Eric Erik", "Eric", "Erik", "hello Erik Erik"},

//                oldStr is at the end
                {"hello Eric Erik", "Erik", "Eric", "hello Eric Eric"},

//                oldStr is at the begining
                {"hello Eric Erik", "hello", "Hello", "Hello Eric Erik"},

//                No oldStr in sourceStr
                {"hello Eric Erik", "H", "E", "hello Eric Erik"},

//                Length of oldStr is more than length of sourceStr
                {"hello Eric Erik", "hello Eric Erik!", "Hello", "hello Eric Erik"},

//                "" cases
                {"hello Eric Erik", "", "H", "hello Eric Erik"},
                {"hello Eric Erik", "", "", "hello Eric Erik"},
                {"hello Eric Erik", "Eric", "", "hello  Erik"},
                {"", "Eric", "Joe", ""},

//                No changes if there is null
                {"hello Eric Erik", null, "Erik", "hello Eric Erik"},
                {"hello Eric Erik", "hello", null, "hello Eric Erik"},
                {null, "hello", "bye", null},
                {null, null, null, null}

        };
    }

    @Test(dataProvider = "replaceAllParameters")
    public void replaceAllTest(String sourceStr, String oldStr, String newStr, String finalStr) {
        assertEquals(replaceAll(sourceStr, oldStr, newStr), finalStr, "Unexpected result: ");
    }

    public String replaceAll(String sourceStr, String oldStr, String newStr) {

        if (sourceStr == null || oldStr == null || newStr == null) return sourceStr;
        if (oldStr.length() == 0) return sourceStr;
        if (sourceStr.length() < oldStr.length()) return sourceStr;

        String result = "";
        int i = 0;
        while (i < sourceStr.length()) {
            if (foundStr(sourceStr, i, oldStr)) {
                result = result + newStr;
                i = i + oldStr.length();
            } else {
                result = result + sourceStr.charAt(i);
                i++;
            }
        }
        return result;
    }

    private boolean foundStr(String source, int i, String str) {
        if (source.length() - i - str.length() < 0) return false;
        for (int j = 0; j < str.length(); j++) {
            if (str.charAt(j) != source.charAt(i + j)) return false;
        }
        return true;
    }

}