Although it's not a good practice, you can concatenate the null value with "" to make it a String.

For example:

String str=null;
System.out.println((str+"").length()); /// prints 4

I would factor out a utility method that captures the intention, which improves readability and allows easy reuse:

public static String blankIfNull(String s) {
    return s == null ? "" : s;
}

Then use that when needed:

System.out.println(blankIfNull(a));

For java 8 you can use Optional approach:

Optional.ofNullable(gearBox).orElse("");
Optional.ofNullable(id).orElse("");

I don't know the context in which you thought to mention streams relating to your question, but if you are open to the Apache StringUtils library, then one option would be to use the StringUtils#isEmpty() method:

if (StringUtils.isEmpty(text)) {
    text = null;
}

tl;dr

set a String to null in Java?

myString = null ;

declare the variable without initializing it

Uninitialized deaults to null, no String object. Empty reference variable.

String myString ;  

You can explicitly assign null. Same effect as line above. No object, empty reference.

String name = null;

Declare and initialize to empty string:

String myString = "" ;  // A `String` object containing no characters. Not null.

Details

Before posting here on basic Java Questions, study the Java Tutorials provided by Oracle free of cost.

See the tutorial page on string literals. To quote:

There's also a special null literal that can be used as a value for any reference type. null may be assigned to any variable, except variables of primitive types. There's little you can do with a null value beyond testing for its presence. Therefore, null is often used in programs as a marker to indicate that some object is unavailable.

👉 So null is not a piece of text with four characters. The keyword null means “no object at all”, no String, nothing at all, an empty reference.

To test if an object reference variable is null (contains no reference), you have a few choices:

• Objects.isNull( myVar ) and Objects.nonNull( myVar )
• null == myVar and null != myVar
• myVar == null and myVar != null

I prefer the first, as words are easier to read than mathematical-like symbols.

Try Objects.toString(Object o, String nullDefault)

Example:

import java.util.Objects;

Object o1 = null;
Object o2 = "aString";
String s;

s = Objects.toString(o1, "isNull"); // returns "isNull"
s = Objects.toString(o2, "isNull"); // returns "aString"