public Tag withKey(String key) {
        setKey(key);
        return this;
    }
<p>
A unique identifier for the tag. Valid characters include Unicode letters, digits, white space, _, ., /, =, +, -,
%, and @.
</p>

@param key
       A unique identifier for the tag. Valid characters include Unicode letters, digits, white space, _, ., /,
       =, +, -, %, and @.
@return Returns a reference to this object so that method calls can be chained together.
aws-java-sdk-machinelearning/src/main/java/com/amazonaws/services/machinelearning/model/Tag.java