public S3ObjectMetadata addUserMetadataEntry(String key, String value) {
        if (null == this.userMetadata) {
            this.userMetadata = new java.util.HashMap<String, String>();
        }
        if (this.userMetadata.containsKey(key))
            throw new IllegalArgumentException("Duplicated keys (" + key.toString() + ") are provided.");
        this.userMetadata.put(key, value);
        return this;
    }
Add a single UserMetadata entry

@see S3ObjectMetadata#withUserMetadata
@returns a reference to this object so that method calls can be chained together.
aws-java-sdk-s3control/src/main/java/com/amazonaws/services/s3control/model/S3ObjectMetadata.java