only-columns.java

List<String> columnNames = task.getColumns();
ImmutableList.Builder<Column> builder = ImmutableList.builder();
int i = 0;
for (String columnName : columnNames) {
    for (Column inputColumn: inputSchema.getColumns()) {
        if (inputColumn.getName().equals(columnName)) {
            Column outputColumn = new Column(i++, columnName, inputColumn.getType());
            builder.add(outputColumn);
            break;
        }
    }
}
Schema outputSchema = new Schema(builder.build());