String statement = "value = alpha*beta + gamma;"  ;
    
    int loc = statement.indexOf( "=");
    if ( loc != -1 )
    {
      String left = statement.substring(  0, loc );
      String right = statement.substring( loc + 1  );
    }

Second Occurrence

A correct line of a Java program only rarely includes two assignment operators. Here is a program fragment that inspects a line for this:

String line = "alpha = beta + 23 = 99;" ;

int spot = line.indexOf( "=" );

if (  spot != -1 )
{
  String newLine = line.substring( spot+1 ) ;
  if ( newLine.indexOf("=") > 0  )
  {
    System.out.println( "Possible Bad Line: " + line ); 
  }
}

The statement

  String newLine = line.substring( spot+1 ) ;

computes the tail of the original line, starting with the character after the first = sign. The clause newLine.indexOf("=") > 0 detects = anywhere but at the beginning of this new line (where it would be OK because it would be part of "==").