Java程序中的延迟加载功能使用

public class DomainObject {
 private final DataParser parser;
 private final Map<String, String> header = new HashMap<>();
 private final List<Data> data = new ArrayList<>();
 
 public DomainObject(DataParser aParser) {
  parser = aParser;
 }
 
 public String getHeaderField(String name) {
  // Here we lazily parse and fill the header map
  if (header.isEmpty()) {
   header.addAll(parser.header());
  }
  return header.get(name);
 }
 public Iterable<Data> getMeasurementValues() {
  // again lazy-load and parse the data
  if (data.isEmpty()) {
   data.addAll(parser.measurements());
  }
  return data;
 }
}

// lazy and fast because the expensive operation will only execute when needed
if (aCondition() && expensiveOperation()) { ... }
 
// slow order (still lazy evaluated!)
if (expensiveOperation() && aCondition()) { ... }

public class DomainObject(parser: DataParser) {
 // evaluated on first access
 private lazy val header = { parser.header() }
 
 def getHeaderField(name : String) : String = {
  header.get(name).getOrElse("")
 }
 
 // evaluated on first access
 lazy val measurementValues : Iterable[Data] = {
  parser.measurements()
 }
}