字符串拼接时数值类型相加引发的问题

public class ClassicPriceSettingReqDto {
    private Integer skuId;
    private Byte skuType;
    private Long classicId;
    private String currency;
    private Long startTime;
    public Integer getSkuId() {
        return skuId;
    }
    public Byte getSkuType() {
        return skuType;
    }
    public Long getClassicId() {
        return classicId;
    }
    public String getCurrency() {
        return currency;
    }
    public Long getStartTime() {
        return startTime;
    }
}

Set<String> existsDataSet = new HashSet<>();
for (ClassicPriceSettingReqDto item : reqDtoList) {
    // 按照指定字段进行去重，前三个字段均为数值类型，而我想要的是字符串的拼接效果
    String str = item.getSkuId() + item.getSkuType() + item.getClassicId() + item.getCurrency() + item.getStartTime() ;
    if (existsDataSet.contains(str)) {
        log.warn("This data already exists {}", str);
        continue;
    }
    existsDataSet.add(str);
}

Set<String> existsDataSet = new HashSet<>();
for (ClassicPriceSettingReqDto item : reqDtoList) {
    // 按照指定字段进行去重
    String str = item.getSkuId() + "-" + item.getSkuType() + "-" + item.getClassicId() + "-" + item.getCurrency() + "-" + item.getStartTime() ;
    if (existsDataSet.contains(str)) {
        log.warn("This data already exists {}", str);
        continue;
    }
    existsDataSet.add(str);
}