Question: I would like to know the code to do this in java please? 

This is what i have so far but it does not work?

```
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import static jdk.nashorn.tools.ShellFunctions.input;

public class Sha256hash 
{

    public static String main(String[] args) throws NoSuchAlgorithmException 
    {
        MessageDigest md = MessageDigest.getInstance("SHA1");
            md.reset();
            byte[] buffer = input.getBytes("UTF-8");
            md.update(buffer);
            byte[] digest = md.digest();

            String hexStr = "";
            for (int i = 0; i < digest.length; i++) {
                hexStr +=  Integer.toString( ( digest[i] & 0xff ) + 0x100, 16).substring( 1 );
            }
            return hexStr;
    }

}

```

Answer: 
I'm still unclear whether you want SHA-1 **or** SHA-256, so let's *abstract* the problem; firstly, an *encode* method to take a `byte[]` and return the hex (don't worry, you already wrote it; but I would prefer a `StringBuilder` over `String` concatenation. Java `String` is **immutable**, so you're creating garbage for later garbage collection with `+`) -

```
private static String encodeHex(byte[] digest) {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < digest.length; i++) {
        sb.append(Integer.toString((digest[i] & 0xff) + 0x100, 16).substring(1));
    }
    return sb.toString();
}

```

Next, we can create a method that takes the *algorithm* name and the `String` to digest and returns that digest. Like

```
public static String digest(String alg, String input) {
    try {
        MessageDigest md = MessageDigest.getInstance(alg);
        byte[] buffer = input.getBytes("UTF-8");
        md.update(buffer);
        byte[] digest = md.digest();
        return encodeHex(digest);
    } catch (NoSuchAlgorithmException | UnsupportedEncodingException e) {
        e.printStackTrace();
        return e.getMessage();
    }
}

```

Then we can get a `SHA-1` **or** a `SHA-256` hash like

```
public static void main(String[] args) {
    System.out.println(digest("SHA1", ""));
    System.out.println(digest("SHA-256", ""));
}

```

Which outputs (as expected)

```
da39a3ee5e6b4b0d3255bfef95601890afd80709
e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855

```
The main entry point can not return String. Furthermore, `input` is not declared. You maybe want to change the name of your function to `generate` with `input` as a parameter.

```
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import static jdk.nashorn.tools.ShellFunctions.input;

public class Sha256hash 
{

    public static String generate(String input) throws NoSuchAlgorithmException 
    {
        MessageDigest md = MessageDigest.getInstance("SHA1");
            md.reset();
            byte[] buffer = input.getBytes("UTF-8");
            md.update(buffer);
            byte[] digest = md.digest();

            String hexStr = "";
            for (int i = 0; i < digest.length; i++) {
                hexStr +=  Integer.toString( ( digest[i] & 0xff ) + 0x100, 16).substring( 1 );
            }
            return hexStr;
    }

}

```

This example works for me returning `c3499c2729730a7f807efb8676a92dcb6f8a3f8f` as result of processing the string `example`:

```
import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class Sha256hash 
{

    public static String generate(String input) throws NoSuchAlgorithmException, UnsupportedEncodingException 
    {
        MessageDigest md = MessageDigest.getInstance("SHA1");
            md.reset();
            byte[] buffer = input.getBytes("UTF-8");
            md.update(buffer);
            byte[] digest = md.digest();

            String hexStr = "";
            for (int i = 0; i < digest.length; i++) {
                hexStr +=  Integer.toString( ( digest[i] & 0xff ) + 0x100, 16).substring( 1 );
            }
            return hexStr;
    }

}

```

Main:

```
import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;

public class Tester {
    public static void main(String[] args) throws NoSuchAlgorithmException, UnsupportedEncodingException {
        String someText = "example";
        System.out.println(Sha256hash.generate(someText));
    }
}

```

Finally, as Elliott has pointed out If you want to use SHA-256 you should change `MessageDigest.getInstance("SHA1");` to `MessageDigest.getInstance("SHA256")`; Right now you are using SHA-1. Also pointed by Elliot you should use `StringBuilder` in the loop for improved efficiency.